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sufficiently to be capable of secreting milk, a point in

time called lactogenesis, Stage I. However, milk secre-

tion is held in check by the high concentrations of

progesterone from the placenta.

Secretory Initiation, Lactation, and Involution

0023 With the birth of the young, the hormonal support for

mammary growth and inhibition of milk secretion are

withdrawn, and secretory initiation begins. This pro-

cess involves the terminal differentiation of the mam-

mary cells through a carefully programmed series of

events that result in copious secretion of milk 4 days

after birth. Milk secretion is maintained until regular

removal of milk from the gland ceases, at which

point, the mammary epithelium begins to involute.

Many of the alveolar cells undergo apoptosis, and the

gland returns to its prepregnant state.

Secretory Initiation

0024 Although the mammary epithelium becomes fully

competent to secrete milk sometime in midpregnancy,

the term secretory initiation is used to describe the

onset of copious milk secretion in the first few days

after parturition and will be so used in the rest of this

discussion. Withdrawal of progesterone and estrogen

in the presence of sustained prolactin secretion brings

about a complex series of well-programmed events

that lead to the secretory activity of differentiated

mammary cells. Immediately after birth, the tight

junctions between the cells begin to close, and the

paracellular pathway is no longer available for the

exchange of components between the milk space and

the interstitial space. The concentrations of sodium

and chloride in the milk fall, and the milk lactose

concentration increases (Figure 4). Next, secretion of

IgA oligosaccharides and lactoferrin increases, adding

large quantities of these protective substances along

with high concentrations of lymphoid cells to the

mammary secretion, now called colostrum. Beginning

about 40 h postpartum, the alveolar cells increase their

secretion of lactose, casein, and the other components

of mature milk, and milk volume production goes

from about 100 ml per day on the second day post-

partum to about 500 ml per day on day 5. The corres-

ponding feeling of fullness of the breast is often

referred to as the ‘milk coming in.’ Suckling by the

infant is not required to initiate the process; however,

milk secretion cannot be maintained if milk is not

removed shortly after the secretory activity increases.

Lactation

0025 At the end of secretory initiation, the breast enters

the stage of lactation, sometimes referred to as

galactopoiesis. This stage persists as long as the infant

continues to suckle at least twice a day. The daily milk

volume transferred to the infant increases on average

from 500 ml on day 5 to about 650 ml at 1 month and

about 750 ml at 3 months. Milk volume is largely

regulated by infant demand. Infants who are larger

at a given age tend to take more milk; if breast milk is

supplemented with other foods or formula, milk

volume decreases. In most women, the breast appears

to have the capacity to secrete much more milk than is

needed for nourishment of a single infant. In fact, wet

nurses who removed milk from their breasts using a

breast pump have been recorded as producing up to

3500 ml per day, and some women are able to breast-

feed triplets!

Regulation of Milk Secretion

0026Prolactin is necessary for milk secretion, and suckling

promotes its secretion. However, the volume of milk

secretion is not directly regulated by the concentra-

tion of prolactin in the blood. Rather, local mechan-

isms within the mammary gland related to the

amount of milk removed by the infant are responsible

for the day-to-day regulation of milk volume. A pro-

tein factor called feedback inhibitor of lactation is

secreted with other milk components into the alveolar

lumen. If milk is not removed from the gland, this

substance, whose identity is not yet entirely clear,

Volume

Lactose

Chloride

Sodium

sIGA

Lactoferrin

120

180

500

250

750

Days postpartum

Concentration

Volume (ml/day)

(grams/liter) (mM)

fig0004Figure 4 Changes in milk composition and volume during

secretory activation in the woman.

LACTATION/Physiology 3463

interacts with the mammary alveolar cell and inhibits

milk secretion possibly by altering the sensitivity of

the cells to prolactin.

Regulation of Milk Ejection

0027 Suckling initiates a neuroendocrine reflex essential

for removal of milk from the breast. Afferent im-

pulses travel via sensory neurons from the areola to

the hypothalamus, where they stimulate magnocellu-

lar neurons to fire, sending an impulse down their

axons into the posterior pituitary where the hormone

oxytocin is released. This hormone travels through

the blood stream to the breast, where it causes con-

traction of the myoepithelial cells, forcing the milk

from the alveoli into the mammary ducts and sinuses

whence it can be removed by the sucking infant.

Without an adequate let-down reflex, complete re-

moval of milk from the breast is not possible. The

activity of the magnocellular neurons can be pro-

foundly influenced by higher brain centers. For this

reason, emotional distress can inhibit let-down. Con-

versely, let-down is subject to conditioning, so that a

woman often releases oxytocin at the sound of her (or

someone else’s) infant’s cry or in response to a picture

of her infant.

Interactions of Lactation with Maternal

Conditions

0028 The lactating breast is largely an autonomous organ,

depending only on permissive levels of prolactin and

other hormones such as insulin and hydrocortisone

necessary for continued milk secretion. The amount

of milk produced is under local control and depends

on the rate of milk removal from the breast, as dis-

cussed above. The gland removes nutrients from the

plasma according to its needs for substrate to synthe-

size milk components and is usually only marginally

affected by acute changes in maternal dietary intake

and other maternal factors such as fluid intake, body

composition, illness, and exercise. The limits to which

lactation is maintained in starvation are unknown,

since experimental observations in this area are diffi-

cult. A single case report suggests that 7 days of total

fasting does not impact milk secretion. The compos-

ition of the diet does, however, affect some milk

components, most notably certain vitamins such as

vitamin C, vitamin A, vitamin B

, vitamin D, and

possibly others. Vitamin B

deficiency has been

reported in infants of mothers consuming a vegetar-

ian diet. The milk levels of some trace elements, for

example, selenium are also affected by maternal diet-

ary status. The effects of the lipid composition of the

diet on milk fat composition have been discussed

above. Fluid intake is not directly linked to milk

production, as women who take no water during

the day during Ramadan produce normal volumes

of slightly hypertonic milk. It is likely that hydration

severe enough to threaten maternal metabolism

would also threaten milk production. The total fat

content of milk does not appear to be affected by

dietary intake but is inversely proportional to body

fat. Diabetics may have difficulty initiating lactation,

but once lactation is established, they have no prob-

lem nursing an infant. Mastitis alters milk compos-

ition by opening the tight junctions, so that the

sodium and chloride content of the milk is increased

and the protein content decreased. Pregnancy anec-

dotally reduces milk volume, although there are no

good data on this point. Exercise has not been ob-

served to alter either milk volume or composition.

0029The impact of lactation on maternal nutritional

and reproductive status is more notable. Exclusive

breast-feeding inhibits the release of gonadotropin-

releasing hormone from the hypothalamus, postpon-

ing the return of reproductive function after birth of

the infant. However, even full breast-feeding does not

necessarily offer protection against a subsequent

pregnancy, particularly in well-nourished women.

Bone calcium has been shown to decline during lacta-

tion in several studies, probably owing to the lack of

estrogen during the period of postpartum amenor-

rhea, but appears to recover to normal values once

the menses have resumed. Women with marginal pro-

tein nutrition may suffer some deterioration of their

protein status. During lactation, the folate stores of

women with marginal folate status may be depleted

of the vitamin, a situation that may have a negative

impact on a subsequent pregnancy. However, com-

pared with species like rodents or dairy animals,

whose body lipid reserves are depleted within a

short time after lactogenesis by their very copious

milk production, metabolic adaptations to lactation

in the woman are relatively small, because feeding a

single infant does not place an enormous metabolic

load on the lactating women compared with rodents

or dairy animals.

See also: Infants: Nutritional Requirements; Breast- and

Bottle-feeding; Lactation: Human Milk: Composition and

Nutritional Value; Pregnancy: Metabolic Adaptations and

Nutritional Requirements

Further Reading

Dewey KG and McCrory MA (1994) Effects of dieting

and physical activity on pregnancy and lactation. Ameri-

can Journal of Clinical Nutrition 59(2 supplement):

446S–452S.

Jensen RG (1995) Handbook of Milk Composition. San

Diego, CA: Academic Press.

3464 LACTATION/Physiology

Lawrence RA (1989) Breastfeeding and medical disease.

Medical Clinics of North America 73: 583–603.

McNeilly AS, Tay CCK and Glasier A (1994) Physiological

mechanisms underlying lactational amenorrhea. Journal

of Mammary Gland Biology and Neoplasia 709: 145–

155.

Neville MC (1989) Regulation of milk fat synthesis. Journal

of Pediatric Gastroenterology and Nutrition 8:

426–429.

Neville MC (2001) Anatomy and physiology of lactation.

Pediatric Clinics of North America 48: 13–34.

Neville MC and Neifert MR (1983) Lactation: Physiology,

Nutrition and Breastfeeding. New York: Plenum Press.

Neville MC and Picciano MF (1997) Regulation of milk

lipid secretion and composition. Annual Review of

Nutrition 17: 159–184.

Newburg DS (1996) Oligosaccharides and glycoconjugates

inhuman milk: Their role in host defense. Journal of

Mammary Gland Biology and Neoplasia 1: 271–284.

O’Connor DL (1994) Folate status during pregnancy

and lactation. Advances in Experimental Medicine &

Biology 352: 157–172.

Peaker M and Wilde CJ (1996) Feedback control of milk

secretion from milk. Journal of Mammary Gland Biol-

ogy and Neoplasia 1: 307–316.

Prentice A (1994) Calcium intakes and bone densities of

lactating women and breast-fed infants in The Gambia.

Advances in Experimental Medicine & Biology 352:

243–255.

Russo J and Russo IH (1987) Development of the human

mammary gland. In: Neville MC and Daniel CW (eds)

The Mammary Gland, pp. 67–96. New York: Plenum

Press.

Sanz AM and Diaz RC (1995) Selenium in human lactation.

Nutrition Reviews 53(6): 159–166.

Specker BL (1994) Nutritional concerns of lactating women

consuming vegetarian diets. American Journal of Clin-

ical Nutrition 59(5 supplement): 1182S–1186S.

Telemo E and Hanson LA (1996) Antibodies in milk.

Journal of Mammary Gland Biology and Neoplasia 1:

243–250.

LACTIC ACID BACTERIA

J A Narvhus, Agricultural University of Norway, Aas,

Norway

L Axelsson, MATFORSK, Norwegian Food Research

Institute, Aas, Norway

Introduction

0001 Lactic acid bacteria (LAB) are responsible for a great

diversification in the flavor and texture of food prod-

ucts due to their fermentation of food raw materials.

However, in some circumstances they can be respon-

sible for food spoilage. Food fermentation technol-

ogy, along with drying and salting, is one of the

most ancient food-preserving techniques known to

humans. Fermented foods are less perishable than

the original raw materials, their nutritional value

may be enhanced, and the safety of these foods may

be improved due to the inhibition of pathogenic bac-

teria by the low pH and the presence of organic acids

and antimicrobial compounds. The ubiquity of LAB

in nature often results in opportunistic inoculation of

food raw materials. Humans have, since prerecorded

times, aided such natural events by purposely adding

a portion of previously fermented food to the new

batch of raw materials (so-called ‘back-slopping’)in

order to promote successful fermentation. In add-

ition, specific traditional technologies often lead to

the dominance of particular types of LAB, which

gives the product special characteristics. (See Fer-

mented Foods: Origins and Applications; Traditional

Food Technology.)

0002Today, many traditional fermentation technologies

have been industrialized and form large commercial

enterprises. Examples of these are the production of

cheese and other fermented dairy products, fermented

sausages such as salami, and fermented cabbage –

sauerkraut. LAB are only weakly proteolytic and

lipolytic, which is also important since breakdown

products from protein and fat are often organolepti-

cally unpleasant in high concentrations. LAB also

inhabit the mucous membranes of the intestinal and

reproductive tracts of humans and animals and in-

corporation of selected strains into certain fermented

foods has gained increasing interest in recent years.

LAB have a reputation of being ‘generally recognized

as safe’ (GRAS). However, some species are patho-

genic and the recent developments in the field of

probiotics have brought safety aspects of LAB into

focus. Aside from the known pathogens, LAB are

rarely isolated from disease and in almost all cases

are associated with underlying illness which increases

the susceptibility of the host.

0003LAB were amongst the very first bacteria studied.

In 1873, Joseph Lister isolated the first bacterial pure

culture which he called Bacterium lactis. This lactic

LACTIC ACID BACTERIA 3465

acid bacterium is now called Lactococcus lactis and is

used for fermenting milk to produce hundreds of

different dairy products. Early work on LAB was

mainly concentrated on those associated with dairy

products and, with time, many commercial starter

cultures were developed. However, LAB were soon

discovered in other habitats and a broader view of the

group developed. From concentrating on the reac-

tions of the organisms in milk, interest was diverted

into more diverse attributes and the taxonomy of the

group was gradually mapped. (See Cheeses: Starter

Cultures Employed in Cheese-making; Fermented

Milks: Types of Fermented Milks; Yogurt: The Prod-

uct and its Manufacture.)

0004 However, the relationships between different LAB

genera have seen many changes. Orla Jensen de-

scribed seven genera in 1919, but only the name of

one of them, Streptococcus, remains valid today. The

LAB group now comprises 16 genera, 12 of which

are associated with food (Table 1). All LAB have a

G þC% under 50. Morphologically, they are cocci,

coccobacilli, or rods and, with the exception of the

tetrad-forming genera Aerococcus, Pediococcus,and

Tetragenococcus, chain formation is common. LAB

are Gram-positive, nonsporing, and most species are

nonmotile. They have complex nutritional require-

ments and are dependent on the presence of a fer-

mentable carbohydrate for active growth. As an end

product of this fermentation, LAB produce copious

amounts of lactic acid either homofermenta-

tively (>95% lactic acid from glucose) or heterofer-

mentatively (producing acetic acid, ethanol, and

carbon dioxide in addition to lactic acid). Under con-

ditions of limiting carbohydrate, fermentation by

homofermentative strains may become mixed. The

group lacks the ability to synthesize porphyrin

groups and they thus have a nonrespiring (cyto-

chrome-negative) metabolism and are catalase-

negative in standard laboratory media. It should be

mentioned here that the genus Bifidobacterium is

often regarded as being a member of the LAB group

as it fits the general description above. In addition,

certain strains of bifidobacteria are used in probiotic

applications, much in the same way as some LAB.

However, bifidobacteria are phylogenetically unre-

lated to LAB and also differ physiologically in that

they utilize a sugar fermentation pathway unique to

the genus. (See Bifidobacteria in Foods.)

0005The separation of the various genera of LAB from

each other using conventional biochemical tests has

always been problematic, since a certain overlapping

of properties exists (Table 1). Nevertheless, the allo-

cation of most LAB isolates to the correct genus

should be possible by the use of the characterization

tests shown in the table.

0006Within each genus, classical division of LABs into

species relies heavily on carbohydrate fermentation

profiles and some other simple physiological tests

such as growth at particular temperatures and toler-

ance of salt and extremes of pH. However, the devel-

opment of genotyping techniques has to a certain

extent revolutionized our understanding of the rela-

tionships within the group. Some of these techniques

can also be used to identify and characterize isolates

down to species and even strain level. Figure 1 shows

the changes in nomenclature within LAB important in

foods, from 1980 to 2000. Figure 2 shows a sche-

matic, unrooted phylogenetic tree of the lactic acid

tbl0001 Table 1 Differential characteristics of lactic acid bacteria

Rods Cocci

Character Carno-

bacterium

Lacto-

bacillus

Aero-

coccus

Entero-

coccus

Lactococcus

Vagococcus

Leuconostoc

Oenococcus

Pedio-

coccus

Strepto-

coccus

Tetrageno-

coccus

Weissella

Tetrad formation þ  þþ 

from glucose



+  þ   þ

Growth at 10



C þ + þþþ þ + þ þ

Growth at 45



C  + þ  ++ 

Growth in 6.5% NaCl ND

+ þþ ++þ +

Growth in 18% NaCl   þ 

Growth at pH 4.4 ND + þ++þ  +

Growth at pH 9.6 þþ  þ 

Lactic acid

LD, L, DL

LL L D L, DL

LL D, DL

þ, positive; , negative; +, response varies between species; ND, not determined.

Weissella strains may also be rod-shaped.

Test for homo-or heterofermentation of glucose; negative and positive denotes homofermentative and heterofermentative, respectively.

Configuration of lactic acid produced from glucose.

Small amounts of CO

can be produced, depending on media.

No growth in 8% NaCl has been reported.

Production of D-, L-, or DL-lactic acid varies between species.

Reproduced from Axelsson L (1998) Lactic acid bacteria: classification and physiology. In: Salminen S and von Wright A (eds) Lactic Acid Bacteria.

Microbiology and Functional Aspects, 2nd edn. New York: Marcel Dekker with permission from Marcel Dekker.

3466 LACTIC ACID BACTERIA

bacteria. Phylogenetically, LAB belong to a cluster of

Gram-positive bacteria which also includes aerobic

and facultative anaerobic genera such as Bacillus and

Staphylococcus. So far, not one genotyping method

has achieved universal acceptance but these methods

are gradually partially replacing the traditional

characterization tests. However, molecular biological

techniques alone have the disadvantage that they give

no information concerning the properties of the or-

ganisms under study. For this reason, studies of the

metabolic properties of LAB, especially with relation

to food product characteristics, are also receiving

increasing attention.

Descriptions of Selected Genera

The Genus

Streptococcus

0007 The genus Streptococcus is one of the original genera

of LAB and was heterogeneous. Taxonomic studies

in the 1980s, however, supported the unofficial sub-

grouping of the genus and concluded that the fecal

and lactic species were sufficiently different to deserve

allocation to their own genera of Enterococcus and

Lactococcus. However, most streptococci are not

food organisms; most are commensals of humans

and animals and, indeed, many are pathogens or

opportunistic pathogens. The notable exception to

this is Streptococcus thermophilus. This species is

characterized by its rapid growth at high tempera-

tures and active fermentation of milk. It is used,

together with Lactobacillus delbrueckii ssp. bulgari-

cus, for the production of yogurt and is found in

many indigenous fermented milks, where it may

occur along with a variety of other LAB and yeasts.

It is also used as a starter culture for a variety of

Italian cheeses, including Parmesan. No other species

of Streptococcus is reported to be important for either

the manufacture or spoilage of food stuffs. (See

Yogurt: The Product and its Manufacture.)

The Genus

Lactococcus

0008Some species of the genus Lactococcus, previously

grouped within Streptococcus and having the group N

Streptococcus Streptococcus

Lactococcus, 1985

Vagococcus, 1989

Enterococcus, 1984

Leuconostoc

Oenococcus, 1995

Weisella, 1993

Lactobacillus

Carnobacterium, 1987

Pediococcus

Tetragenococcus, 1990

Aerococcus

Leuconostoc

Lactobacillus

Pediococcus

Aerococcus

fig0001 Figure 1 Genera of lactic acid bacteria important in food, showing changes in nomenclature from 1980 to 2000.

LACTIC ACID BACTERIA 3467

antigen, are extremely important in the dairy industry.

They can be isolated from spontaneously fermented

milks and, more importantly, are an essential part of

commercialdairy starterculturesusedforthemanufac-

ture of fermented milks, sour cream, fermented butter,

unripened cheeses (such as cottage and cream cheeses),

and also for a multitude of vastly varying matured

cheeses. Lactococci are often used in coculture with

Leuconostoc strains. L. lactis is the most commercially

important Lactococcus. It has two subspecies – cre-

moris and lactis – and the latter subspecies has a biovar-

iant–diacetylactis.Thetwosubspecies differ from each

other in a variety of biochemical characterisation tests;

the subspecies cremoris is markedly less reactive than

lactis but it has the reputation of being a better flavor

producer in cheese. In the nondairy environment,

strains of lactis are isolated more frequently than

cremoris. Some strains of cremoris form extracellular

polysaccharides and are found in traditional Scandi-

navian ropy milks. (See Cheeses: Starter Cultures

Employed in Cheese-making; Fermented Milks:

Types of Fermented Milks; Starter Cultures.)

0009The biovariant diacetylactis deserves a special men-

tion since its one distinguishing feature – the ability to

metabolize citrate – results in the formation of

diacetyl in fresh fermented milk products and in the

production of carbon dioxide which forms eyes in

Gouda-type cheeses. Other species of Lactococcus

are not considered important in the food industry.

The Genus

Vagococcus

0010The genus Vagococcus is phenotypically indistin-

guishable from lactococci and also possesses the

group N antigen. However, vagococci are often

motile; they have been isolated from chicken feces,

water, and diseased fish.

The Genus

Enterococcus

0011Cocci which are Gram-positive, catalase-negative,

possess the group D antigen, and are able to grow in

Lactobacillus delbrueckii group

Lactobacillus

casei-Pediococcus

group

Lactococcus

Streptococcus

Bacillus

Staphylococcus

Listeria

Aerococcus

Carnobacterium

Enterococcus

Vagococcus

Tetragenococcus

Oenococcus

Leuconostoc

Weissella

fig0002 Figure 2 Schematic, unrooted phylogenetic tree of the lactic acid bacteria, including some aerobic and facultative anaerobic Gram-

positives of the low GþC subdivision. Note: evolutionary distances are approximate. Reproduced from Axelsson L (1998) Lactic acid

bacteria: classification and physiology. In: Salminen S and von Wright A (eds) Lactic Acid Bacteria. Microbiology and Functional Aspects,

2nd edn. New York: Marcel Dekker with permission from Marcel Dekker.

3468 LACTIC ACID BACTERIA

6.5% salt and in the presence of 0.4% sodium azide,

can be identified presumptively as enterococci. This

genus was previously grouped within Streptococcus.

Certainly all the classical species of Enterococcus fall

into this category, but some newly described species

have some aberrant properties which has resulted in

the unofficial division into five subgroups. Several

species of Enterococcus are naturally found in the

intestinal tract of humans and animals. The role of

enterococci in food is unclear and much debated.

These organisms have been isolated from many indi-

genous fermented foods, but their possible positive

contribution has not been elucidated since such prod-

ucts commonly contain several species of LAB. The

tolerance of members of the genus for extremes of

pH, salt, and temperature makes their survival in

fermented foods unproblematic. The fecal original

of many species, such as E. faecalis and E. faecium,

however, means that their presence in foods is

regarded as being an indication of poor hygiene. Fur-

ther complications are the opportunistic pathogeni-

city of some species of Enterococcus, their ability

to decarboxylate amino acids and thereby produce

amines in foods, and finally their propensity to be

resistant to antibiotics and to transfer such traits by

means of mobile genetic elements. A new area of

debate concerns the advisability of using these organ-

isms as probiotics.

The Genus

Leuconostoc

0012 All leuconostocs are heterofermentative cocci. Their

main habitat is on plants and, to a lesser extent, in

meat, milk, and dairy products. As in other genera of

LAB, the member species in this genus are continually

being revised, but this is a comparatively small genus,

with about 11 species. The most common species are

Leuconostoc mesenteroides, which has three subspe-

cies: mesenteroides, cremoris,anddextranicum, and

Leuc. lactis. Several other species at present placed in

the genus Leuconostoc show poor genotypic hom-

ology with the above species, and also with each

other. Leuconostocs play an important role in the

early stages of the fermentation of vegetable products

such as dill pickles, olives, and sauerkraut. They take

over from the initial, mainly Gram-negative flora and

are important for the development of the characteris-

tic flavor of the products. Later in the fermentation

other more aciduric LABs dominate.

0013 Several leuconostocs produce extracellular poly-

saccharides which can be used as food-thickening

agents or stabilizers. However, these dextrans cause

problems in sugar factories and also in fermented

sausages, where the production of slime is undesir-

able. Some mesophilic dairy starter cultures con-

tain leuconostocs. They produce flavor compounds

(diacetyl and acetate) from the metabolism of citrate

and reduce acetaldehyde to ethanol. In addition, pro-

duction of CO

from both citrate and carbohydrates

is important for the eyes in Gouda-type cheeses and

the slight effervescence in cultured buttermilk.

The Genus

Oenococcus

0014Oenococcus oeni (previously Leuconostoc oenos)is

the only member of this new genus. It is found in wine

and is responsible for the malo-lactic fermentation

which occurs during the maturation of wine. O.

oeni converts l-malic acid to l-lactic acid, thus in-

creasing the pH by 0.1–0.3 pH units, and rendering

the wine microbiologically stable. Traditionally pre-

sent as a natural contaminant in wine, this organism

may be purposely introduced to some wines to insure

the desired maturation.

The Genus

Lactobacillus

0015Members of the genus Lactobacillus are rod-shaped

and number over 60 species. Despite the fact that

several species have recently been removed to two

new genera (Weissella and Carnobacterium), the

genus is still heterogeneous. Lactobacilli are found

in a variety of habitats and vary considerably in

their characteristics. In 1919, Orla-Jensen grouped

the lactobacilli into three subgenera: Thermobacteria,

Streptobacteria,andBetabacteria. These subgenera

are currently not taxonomically valid, but this sub-

division is still useful to divide the genus into three

primary phenotypic groups: the obligately homo-

fermentative; the facultatively heterofermentative;

and the obligately heterofermentative.

0016Lactobacilli are naturally found in nutritive and

carbohydrate-rich habitats such as plants or material

of plant origin, meat, milk, and dairy products. They

are also found in manure, silage, and sewage. The

mucous membranes (intestinal, vaginal, and oral) of

humans and animals are often dominated by lacto-

bacilli, where they are thought to have a probiotic

function. (See Microflora of the Intestine: Probiotics.)

0017In the food industry, lactobacilli may be respon-

sible for the desired fermentation of some products

but also for the spoilage of others. In dairy foods, Lb.

delbrueckii ssp. bulgaricus is used as a starter culture

for yogurt and a variety of cheeses, particularly

those manufactured using a high cooking tempera-

ture. During the ripening of most cheeses, so-called

‘non-starter’ lactobacilli (NSLAB) develop and are

important for the development of the cheese flavor.

Present research focuses on identifying useful strains

with the prospect of greater control over cheese

ripening. Heterofermentative lactobacilli may cause

unwanted openness in some cheeses due to CO

production from residual carbohydrate.

LACTIC ACID BACTERIA 3469

0018 Lactobacilli play an important role in the fermen-

tation of many cereal products, such as sourdough,

Tanzanian togwa and Nigerian ogi, where they pro-

duce certain flavor compounds and reduce the pH to

as low as 3.5. Several species of lactobacilli are natur-

ally found in fermented sausages such as salami, and

commercial starter cultures for such products com-

monly contain Lb. plantarum. However, they may

also cause the spoilage of refrigerated fresh and

cooked meats kept in a CO

-enriched atmosphere.

Although this may inhibit the growth of other spoil-

age organisms and also pathogens, defects such as

slime, green color, gas, and acidic taste may develop.

Various fermented vegetables also contain lactoba-

cilli. The early stages are dominated by leuconostocs

and/or heterofermentative lactobacilli, but the final

product is usually dominated by more acid-tolerant,

homofermentative species.

The Genus

Carnobacterium

0019 The genus Carnobacterium is comparatively new,

comprising some former lactobacilli which do not

grow on acetate agar or at pH 4.5. Phylogenetically,

however, they are most closely related to enterococci.

They have so far been isolated from five habitats:

meat (and meat products), cheese, fish, poultry, and

sea water. Most reports are of isolates from meat

that is stored vacuum-packed or in a CO

/nitrogen

atmosphere at refrigeration temperatures.

The Genus

Weissella

0020 Weissella was created when some aberrant species of

Leuconostoc and heterofermentative Lactobacillus

were found to be genotypically related. Species within

this genus have been isolated from habitats such as

meat and dairy products, sewage, and plants.

The Genera

Pediococcus, Aerococcus

, and

Tetragenococcus

0021 Pediococci, along with aerococci and tetragenococci,

divide in two perpendicular directions (often wrongly

described as ‘dividing in two planes’), and thereby

form tetrads of cells rather than chains. Tetrageno-

coccus contains strains formerly regarded as P. halo-

philus. This species is characterized by its extreme

tolerance to salt (> 18%) and is important in the

fermentation of high-salt-containing foods such as

soy sauce. Aerococci are of minor interest in food

technology and will not be further described here.

Pediococci are found, along with other LAB in fer-

mented vegetables, meat and fish. P. damnosus (also

called P. cerevisciae) is of considerable nuisance in the

brewing industry where it can cause ‘ropy’ beer due to

capsular material and flavor defect due to diacetyl

production. Pediococci do not grow well in milk but

can be isolated from milk and dairy products. They

may possibly be significant in the maturation of

cheese where they can be found as part of the

NSLAB community.

Commercial Applications of LAB in the

Food Industry

0022The occurrence of specific LAB in various fermented

foods has been outlined above. It is important to

emphasize that there is a great difference in the degree

of sophistication in the use of these bacteria. As qual-

ity control regulations for all foodstuffs become

more stringent, the demand for control of fermented

foods is also increasing. Whilst in the dairy industry

commercial starter cultures have been available for

many decades, their introduction into other fer-

mented food products is either relatively new or is

not yet in evidence. (See Starter Cultures.) This applies

in particular to the countless varieties of fermented

foods produced at artisan level in developing coun-

tries. Many of these products have not yet

been scientifically described and the microorganisms

responsible for the fermentation processes have not

been identified. Despite considerable research into

the metabolism of LAB, details of their various im-

portant metabolic activities in well-known products

are only partially mapped.

0023The use of starter cultures in the dairy industry is a

special case since milk pasteurization results in the

destruction of the indigenous LAB. To achieve a suc-

cessful fermentation it is therefore necessary to add

suitable cultures of LAB. The development of the

commercial starter culture industry has led to greatly

increased quality and stability of dairy products and

also for the few other fermented foods where they are

utilized.

0024There is currently considerable research on so-

called ‘indigenous’ fermented foods. These products

are produced by natural fermentation and usually the

responsible organisms create a complex mixture of

various LAB, sometimes also interacting with other

bacteria and/or fungi. Present studies are gradually

elucidating the LAB present in these products, their

interactions, and attempting to identify them. Al-

though many of the strains isolated from these prod-

ucts resemble known species of LAB, they often have

special properties which may make them a useful

source of gene material for modifying known com-

mercial cultures.

Bacteriophage Infection of Cultures

0025In 1935, fermentation problems in the dairy industry

were diagnosed as being due to virus particles –

3470 LACTIC ACID BACTERIA

bacteriophages – attacking and destroying the starter

culture. This has remained one of the most persistent

problems in the dairy industry, despite the introduc-

tion of many innovative technological procedures to

prevent their introduction to the process, and despite

the wealth of research into phage–host interactions

and phage resistance systems. The mode of action of

phages, at a molecular level, is now known and the

future will probably see the development of commer-

cially available cultures that have been engineered to

be resistant to phage attack. This methodology will

also be of use in the future development of new starter

cultures for controlled fermentations of other prod-

ucts where the availability of a selection of strains

showing identical fermentation properties, but

different phage susceptibility, may be problematic.

Instability of Starter Cultures

0026 Variation in the properties of starter cultures has been

a problem since their introduction, and has two main

causes.

0027 Natural, heterogeneous starter cultures, often pro-

duced by commercialized back-slopping techniques,

show a certain variation in their fermentation proper-

ties from production to production. This is due to the

variation in the balance of strains in the mixture,

which may be caused by differences in food raw

materials or culture media, in fermentation condi-

tions or by interreactions between the composite

strains. Since the different strains present in the cul-

ture may well have specific roles to play in product

characteristics, an imbalance between the strains will

result in unstable product quality.

0028 Many LAB bear some of their genetic material on

extracellular DNA particles called plasmids. Plasmids

in lactococci have been extensively studied and they

have also been found in many other LAB. Although

the function of many plasmids is cryptic, in lactococci

they code for technologically important properties

such as bacteriophage resistance mechanisms, protei-

nase activity, and the ability to break down lactose and

citrate. Loss of plasmids may occur during cell

division, and this naturally results in the loss of the

specific gene and the culture becomes commercially

useless. In the commercial preparation of starter

cultures the propagation conditions are carefully

controlled so as to minimize the risk of plasmid loss.

Bacteriocins from LAB

0029 Strains from almost all genera of LAB have been

shown to produce many different proteinaceous anti-

microbial substances, collectively called bacteriocins.

Most of these are cationic, amphipathic peptides and

will insert into the cell membrane of closely related

bacteria, thereby leading to inhibition of growth by

pore formation. The best known bacteriocin is nisin,

and this is used as a food preservative in more than 50

countries. (See Nisin.) The 1990s have seen consider-

able research in this field, aimed at finding bacterio-

cins active against foodborne pathogens. Many

bacteriocins have been thoroughly characterized and

the genetics of their biosynthesis has been elucidated.

However, commercial application of these com-

pounds – or the bacteria producing them – has,

apart from nisin, so far not materialized to any large

extent.

Probiotic LAB

0030Since Metchnikoff’s work in 1908, the LAB normal

microbial flora of the body has been believed to have

a beneficial effect on the well-being of the host. There

is increasing interest worldwide for developing food

products which contain strains of LAB which have

been isolated from the human gut – so-called probio-

tic products. The majority of products on the market

are fermented dairy products, and some fermented

cereals have been developed. The commercial poten-

tial of such products is dependent upon the availabil-

ity of convincing scientific documentation of the

positive effect of ingesting these organisms. When

based on well-designed double-blind experiments,

there have been many recent reports showing signifi-

cant effect on the balance of the gut microflora and

also some that indicate that probiotic bacteria can

promote a general stimulation of the immune system.

Most countries demand documented proof of the

efficacy of such products before claims can be made

in promotion advertising. (See Acidophilus Milk;

Bifidobacteria in Foods; Probiotics.)

Genetic Modification of LAB

0031Most LAB can now be genetically modified through

recombinant DNA technology, although the degree

of advanced modification that can be done varies

between species and strains. For Lactococcus lactis,

the most studied and probably the most economically

important species, the available techniques are very

advanced indeed. Essentially any protein of choice

can be highly expressed in a controlled way, and this

can be done by chromosomal integration in a so-

called food-grade manner (i.e., no antibiotic resist-

ance markers or other unwanted genetic elements

are used in the procedure). It is now possible to

change the properties of starter strains, for instance

aroma formation capability or enhanced antimicro-

bial activity against pathogens. The introduction of

LACTIC ACID BACTERIA 3471

such strains into the market depends more on legisla-

tion and consumer acceptance than the construction

of the strains. It should be noted that some properties

can be transferred between strains by classical genetic

methods, e.g., conjugation, and that this is still an

important tool in starter development since these

techniques are unproblematic from a legislative and

consumer point of view.

See also: Acidophilus Milk; Bifidobacteria in Foods;

Cheeses: Starter Cultures Employed in Cheese-making;

Fermented Foods: Origins and Applications; Fermented

Milks: Types of Fermented Milks; Microflora of the

Intestine: Probiotics; Nisin; Probiotics; Starter

Cultures; Traditional Food Technology; Yogurt: The

Product and its Manufacture