FOOD MICROBIOLOGY, A MICROBE JOURNAL.
BASIC FOOD MICROBIOLOGY.
1.1 Introduction
Food science
is a discipline concerned with all aspects of food beginning after harvesting,
and ending with consumption by the consumer. It is considered one of the
agricultural sciences, and it is a field which is entirely distinct from the
field of nutrition. The field of food microbiology is a very broad one, encompassing
the study of microorganisms which have both beneficial and deleterious effects
on the quality and safety of raw and processed foods. It is important to
understand the relationships among the various microorganisms making up the
microflora of a food. In fact, food microbiologists are concerned with the
practical implications of the microflora of the food and the food
microorganisms that can cause spoilage of food and disease in humans. The
primary tool of microbiologists is the ability to identify and quantitate
food-borne microorganisms. However, the inherent inaccuracies in enumeration
processes, and the natural variation found in all bacterial populations
complicate the microbiologist’s job. Moreover, they may be important from the
aesthetic point of view. Of course, some useful bacteria may be important
because they change the functional properties of food stuffs resulting in new
tastes, odors or textures. Microorganisms in food include bacteria, molds,
yeasts, algae, viruses, parasitic worms and protozoa. These organisms differ in
size and shape and in their biochemical and cultural characteristics.
The
microorganisms described below are among the most important genera and species
normally found in food products. Each microorganism has its own particular
nutritional and environmental requirements.
1.2 Bacteria
1.2.1 Acinetobacter
Acinetobacter is a genus of Gram-negative bacteria belonging to the
Gammaproteobacteria . Acinetobacter species are non-motile and
oxidase-negative, and occur in pairs as observed under magnification. Young
cultures show rod shaped morphology. They are strict aerobes that do not reduce
nitrates. They are important soil and water organisms and are also found on
many foods especially refrigerated fresh products. A. baumannii is
a frequent cause of nosocomial pneumonia, especially of late onset ventilator
associated pneumonia. It can cause various other infections including skin and
wound infections, bacteremia, and meningitis,
1.2.2 Bacillus
cereus
B.
cereus is a thick long rod shaped
Gram positive, catalase positive aerobic spore former and the organism is
important in food borne illness. It is a normal inhabitant of soil and is
isolated from a variety of foods. It is quite often a cause of diarrheal
illness due to the consumption of desserts, meat, dishes, dairy products, rice,
pasta etc that are cooked and kept at room temperature as it is thermoduric.
Some of the B. cereus strains are psychrotrophic as they grow
at refrigeration temperature.
B. cereus is spread from soil and
grass to cows udders and into the raw milk. It is also capable of establishing
in cans. It is also capable of producing proteolytic and amyloltic enzymes and
also phoslipase C (lecithinase). The production of these enzymes by these organisms
can lead to the spoilage of foods. The diarrheal illness is caused by an
enterotoxin produced during the vegetative growth of B. cereus in
small intestine. The bacterium has a maximum growth temperature around 48°C to
50°C and pH range 4.9 to 9.3. Like other spores of mesophilic Bacillus species,
spores of B. cereus are also resistant to heat and survive
pasteurization temperature.
1.2.3 Bacillus
subtilis
Bacillus subtilis , known also as the hay bacillus or grass bacillus , is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is thin short rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate extreme environmental conditions. B. subtilis produces the proteolytic enzyme subtilisin. B. subtilis spores can survive the extreme heat during cooking. B. subtilis is responsible for causing ropiness a sticky, stringy consistency caused by bacterial production of long-chain polysaccharides in spoiled bread dough. A strain of B. subtilis formerly known as Bacillus natto is used in the commercial production of the Japanese food natto, as well as the similar Korean food cheonggukjang. It is used to produce amylase and also used to produce hyaluronic acid, which is useful in the joint-care sector in healthcare.
1.2.4 Carnobacterium
Carnobacterium is a genus of Gram-positive bacteria within the family Leuconostocaceae. C.
divergens and C. maltaromaticum are found in the wild
and in food products and can grow anaerobically. These species are not known to
be pathogenic in humans but may cause disease in fish. The genus Carnobacterium contains
nine species, but only C. divergens and C. maltaromaticum are
frequently isolated from natural environments and foods. They are tolerant to
freezing/thawing and high pressure and able to grow at low temperatures,
anaerobically. They metabolize arginine and various carbohydrates, including
chitin, and this may improve their survival in the environment. Carnobacterium
divergens and C. maltaromaticum have been extensively
studied as protective cultures in order to inhibit growth of Listeria
monocytogenes in fish and meat products. Several carnobacterial bacteriocins
have been identified and described. Carnobacteria can spoil chilled foods, but
spoilage activity shows intraspecies and interspecies variation. Their
production of tyramine in foods is critical for susceptible individuals, but
carnobacteria are not otherwise human pathogens.
1.2.5 Corynebacterium
Corynebacterium is a genus of Gram-positive rod-shaped bacteria. They are widely
distributed in nature and are mostly innocuous. Some are useful in industrial
settings such as C. glutamicum. Others can cause human
disease. C. diphtheriae, for example, is the pathogen responsible
for diphtheria. Some species are known for their pathogenic effects in humans
and other animals. Perhaps the most notable one is C. diphtheriae,
which acquires the capacity to produce diphtheria toxin only after interacting
with a bacteriophage. Diphtheria toxin is a single, 60,000 molecular weight
protein composed of two peptide chains, fragment A and fragment B, held
together by a disulfide bond.
1.2.6 Clostridium
perfringens
C.
perfringens is a
Gram-positive encapsulated anaerobic non-motile bacterium commonly found on
meat and meat products. It has the ability to cause food borne disease. It is a
toxin producing organism-produces C. perfringens enterotoxin
and β -toxin that are active on the human GI tract.
It multiplies
very rapidly in food (doubling time < 10 min). Spores are resistant to
radiation, desiccation and heat and thus survive in incompletely or
inadequately cooked foods.
However, it tolerates moderate exposure to air.
Vegetative cells of C. perfringens are also somewhat heat
tolerant as they have relatively high growth temperature (43°C -45 °C ) and can
often grow at 50°C. They are not tolerant to refrigeration and freezing. No
growth occurs at 6 °C . C. perfringens is present in soil and
the other natural environment.
1.2.7 Clostridium
botulinum
C. botulinum produces the most potent
toxin known. It is a Gram-positive anaerobic rod shaped bacterium. Oval endospores
are formed in stationary phase cultures. There are seven types of C.
botulinum (A to G) based on the serological specificity of the
neurotoxin produced. Botulism is a rare but very serious disease. The ingestion
of neurotoxin produced by the organism in foods can lead to death. However, the
toxin (a protein) is easily inactivated by heat. The organism can grow at
temperature ranging from 10-48 °C with optimum growth temperature at
37°C. Spores are highly heat resistant. The outgrowth of spores is inhibited at
pH < 4.6, NaCl > 10% or water activity < 0.94. Botulinum spores are
probably the most radiation resistant spores of public health concern.
Contamination of foods is through soil and sediments where they are commonly
present. The organism grows under obligate anaerobic conditions and produces
toxin in under processed (improper canning) low acid foods at ambient
temperature.
1.2.8 Campylobacter
Campylobacter
are Gram negative nonspore
forming rods. Campyloleacter jejuni is an important food borne
pathogen. It is one of the many species within the genus Campylobacter. Campylobacter species C.
jejuni and C. coli cause diarrhea in humans. The
organism is heat sensitive (destroyed by milk pasteurization temperature). It
is also sensitive to freezing. The organism belongs to the family Campylobactereaceae.
The organisms are curved, S-shaped, or spiral rods that may form spherical or
coccoids forms in old cultures or cultures exposed to air for prolonged
periods. Most of the species are microaerophilic. It is oxidase and catalase
positive and does not grow in the presence of 3.5% NaCl or at 25 °C or below.
The incidence reported for gastro enteritis by this organism are as high as in
case of Salmonella.
The organism is commonly present in raw milk,
poultry products, fresh meats, pork sausages and ground beef. The infective
dose of C.jejuni may be <1,000 organisms.
1.2.9 Erwinia
Erwinia is a genus of the family Enterobacteriaceae bacteria
containing mostly plant pathogenic species. The organisms was named after the
first phytobacteriologist, Erwin Smith. It is a Gram negative bacterium related
to E. coli, Shigella, Salmonella and Yersinia.
It is primarily a rod-shaped bacterium. A well-known member of this genus is
the species E. amylovora, which causes fire blight on apple, pear
and other Rosaceous crops. Erwinia carotovora (also known
as Pectobacterium carotovorum) is another species, which causes
diseases in many plants. These species produce pectolytic enzymes that
hydrolyze pectin between individual plant cells. . Decay caused by E.
carotovora is often referred to as bacterial soft rot (BSR). Most
plants or plant parts can resist invasion by the bacteria, unless some type of
wound is present. High humidity and temperatures around 30°C favor development
of decay.
1.2.10 Enterococcus (E.
faecium, E. faecalis )
Enterococcus is a genus of lactic acid bacteria. Enterococci are Gram positive
cocci that often occur in pairs (diplococci) or short chains and are difficult
to distinguish from streptococci on physical characters mentioned above. The
two species are commensal organisms in the intestine of humans.
The Enterococci are facultative anaerobic
organisms non spore forming that grows optimally at 35°C . However, they
tolerate wide range of environmental conditions (10-45°C) pH (4.5 to 10.5)
quite high NaCl concentration (.6.5%) and can survive heating at 60°C for 30
min.
Catalase-negative,
oxidase negative-bacteria of the genes Enterococcus are
ubiquitous organisms that often occur in large numbers on vegetables, plant
materials and foods especially those of animal origin such as meat and dairy
products. Enterococci also constitute a large preparation of autochthonous
bacteria associated with the mammalian gastro-intestinal tract.
The
resistance of enterococci to pasteurization temperatures and their adaptability
to different substrates and growth conditions in food products manufactured
from raw materials and in heat treated food products is of great significance.
Enterococci
may constitute an important part of the microflora of fermented cheese and
meats.
1.2.11 Escherichia
coli
E. coli strains are associated with food borne gastroenteritis. These are
Gram-negative asprogeneous rods that ferment lactose and produce dark colonies
with a metallic sheen on Endo agar. The organism grows well on a large number
of media and in many foods. They grow over a wide range of temperature (4 to 46
°C ) and pH (4.4 to 9.0).
However, they
grow very slowly in foods held at refrigerator temp. (5 °C ). They belong to
the family Enterobacteriaceae. The organism is also an indicator of
fecal pollution. The organism is also capable of producing acid and gas and
off-flavours in foods. E. coli strains involved in
foodborne-illness can be placed into five groups: enteropathogenic (EPEC),
enterotoxigenic (ETEC), enteroinvasive (EIEC), enterohemorrhagic (EHEC) and facultatively
enteropathogenic (FEEC).
The organism also grows in the presence of bile
salts. The primary habitat of E.coli is the intestinal tract
of most warm blooded animals. E.coli 0157: H7 strains are
unusually tolerant of acidic environments.
1.2.12 Lactococcus
L.lactis subsp. lactis
L.lactis subsp. cremoris
L.lactis subsp.lactis biovar diaectylactis
Lactococcus is a genus of lactic acid bacteria that were formerly
included in the genus Streptococcus Group N (Group N Streptococci). They are
known as homofermentors meaning that they produce a single product of glucose
fermentation. They are Gram-positive, catalase negative, non-motile coccus that
are found singly, in pairs or in chains. Some of the strains of lactococci are
known to grow at or below 7 °C.
Lactococci
are intimately associated with dairy products. These organisms are commonly
used in the dairy industry in the manufacture of fermented dairy products like
cheeses. They can be used in single strain starter cultures or in mixed strain
cultures with other lactic acid bacteria such as Lactobacillus and Streptococcus.
Their main purpose in dairy production is the rapid acidification of milk. This
causes drop in the pH of fermented product which prevents the growth of
spoilage and pathogenic bacteria. These bacteria also play a role in the flavor
of the final product. Dairy lactococci have also been exploited for several
industrial fermentations in the biotechnology industry. They are easily grown
at industrial scale up on cheap whey based media.
Lactococcus lactis subsp. lactis includes
species formerly designated as S. lactis subsp. lactis. L.
lactis subsp. cremoris is distinguished from L.
Lactis subsp. lactis by the inability to (i) grow at
40 °C (ii) grow in 4% NaCl (iii) hydrolyse arginine and (iv) ferment
ribose.
1.2.13 Lactobacillus (L. bulgaricus, L. helveticus. L. plantarum, L. acidophilus, L.
casei, L. lactis, L. fermentum)
The organisms
belonging to this important genus are rods usually long and slender and in some
of the species form chains. They are aerotolerant/microaerophilic but some
ferment sugars chiefly to lactic acids if they are homofermentative. The hetero
fermentative species, besides lactic acid, also produce small amount of acetic
acid, carbon dioxide and trace amounts of volatile compounds such acetaldehyde
and alcohol. The homofermentative species of Lactobacillus include L.
bulgaricus, L. casei, L. helveticus, L. lactis, L. acidophilus and
grow optimally at 37 °C. L. fermentum, L. brevis are
the typical example of hetero fermentative Lactobacillus and
grow well at higher temperatures.
Lactobacilli
are of considerable importance in foods as they ferment sugar to lactic acid
and other desirable flavouring compounds and are thus used in the production of
fermented plant dairy and meat products. However, they are also implicated in
the spoilage of wine and beer.
The organism
normally occurs on plant surfaces silage, manure and dairy products. They are
quite fastidious in their nutritional requirements as they are unable to
synthesize certain vitamins they require and, therefore, media need to be
supplemented with these vitamins for their growth.
Some of the strains are psychotrophic in nature
and are thus involved in the spoilage of refrigerated meats. On the other hand
thermoduric properties (resistance to pasteurization temperature) of some of
the thermophilic strains of lactobacilli are quite useful in the manufacture of
certain varieties of cheeses e.g. Swiss cheese. Some strains of lactobacilli
also show probiotic attributes and are finding application in functional
probiotic foods and in pharmaceutical preparations.
1.2.14 Leuconostoc
Leuconostoc is a genus of Gram-positive bacteria, placed within the family
of Leuconostocaceae. They are generally ovoid cocci often forming
chains. Leuconostoc spp. are intrinsically resistant to
vancomycin and are catalase-negative (which distinguishes them from
staphylococci). All species within this genus are heterofermentative and are
able to produce dextran from sucrose. They are generally slime-forming. Blamed
for causing the 'stink' when creating a sourdough starter, some species are
also capable of causing human infection.
Leuconostoc spp. along with other lactic acid bacteria such as Pediococcus and Lactobacillus spp
, is responsible for the fermentation of cabbage, to sauerkraut. In this
process the sugars in fresh cabbage are transformed to lactic acid which give
it a sour flavour and good keeping qualities.
1.2.15 Listeria
monocytogenes
Listeria
monocytogenes in foods has
attracted worldwide attention due to the serious illness it causes in human
beings. The Listeria are Gram positive non spore forming, nonacid-fast rods.
The organism is catalase positive and produces lactic acid from glucose and
other fermentable sugars. The organism grows well in brain heart infusion
(BHI), trypticase soy, and tryptose broths. However, the medium should be
fortified with B. vitamins and the amino acids. It is a mesophilic organism
with optimal growth temperature 37°C but it can grow at refrigerator
temperature also. Strains grows over the temperature range of 1°C to 45°C and
pH range 4.1 to 9.6.
Listeria
monocytogenes is widely
distributed in nature and can be isolated from decaying vegetation, soil,
animal feces, sewage, silage and water. The organism has been found in raw
milk, pork, raw poultry, ground beef and vegetables. The HTST treatment of
pasteurization is good enough to destroy the organism in milk.
The most
significant virulence factor associated with L. monocytogenes is
listeriolysin O. The virulent strains produce β-hemolysis on blood agar and
acid from rhamnose.
L.
monocytogenes grows well in
moderate salt concentrations (6.5%).
L.
monocytogenes is unique among
foodborne pathogens while other pathogens excrete toxins or multiply in the
blood stream, L. monocytogenes enters the host’s cells and
grows inside the cell. In humans it crosses the intestinal barrier after
entering by the oral route.
Ready to Eat (RTE) foods that are preserved by
refrigeration pose a special challenge with regard to L. monocytogenes infection.
1.2.16 Micrococcus
Micrococcus occurs in a wide range of environments, including water, dust, and
soil. Micrococci are Gram-positive spherical cells ranging from about 0.5 to 3
micrometers in diameter and typically appear in tetrads. Micrococcus has
a substantial cell wall, which may comprise as much as 50% of the cell mass.
Some species of Micrococcus, such as M. luteusM. roseus (red)
produce yellow or pink colonies when grown on mannitol salt agar. Micrococcus is
generally thought to be a saprophytic or commensal organism, though it can be
an opportunistic pathogen, particularly in hosts with compromised immune
systems, such as HIV patients.
1.2.17 Proteus
Since it
belongs to the family of Enterobacteriaceae, general characters are
applied on this genus: It is oxidase-negative, but catalase and nitrate
reductase positive. Three species P. vulgaris, P. mirabilis,
and P. penneri are opportunistic human pathogens. Proteus includes
pathogens responsible for many human urinary tract infections. P.
mirabilis causes wound and urinary tract infections. Most strains
of P. mirabilis are sensitive to ampicillin and
cephalosporins. P. vulgaris is not sensitive to these
antibiotics. However, this organism is isolated less often in the laboratory
and usually only targets immune suppressed individuals. P. vulgaris occurs
naturally in the intestines of humans and a wide variety of animals; also
manure, soil and polluted waters. P. mirabilis, once attached to
urinary tract, infects the kidney more commonly than E. coli. P.
mirabilis are often found as free-living organisms in soil and water.
1.2.18 Propionibacterium spp.
(P. freudenreichii)
Historically, Propionibacterium spp.
are of interest because of their use as dairy starters (especially in the
production of Swiss-type cheese) and their ability to produce propionic acid
during growth. The genus Propionibacterium is generally split
into “cutaneous” and “dairy” groups. The dairy Propionibacterium spp.
can also be isolated primarily from dairy foods and silage. The species in
dairy products include P. jensenii, P. acidipropionici, P. theoniiP.
freudenreichii. Propionibacteria have a role in the production of flavour
compounds in cheese by proteolysis and propionic acid production. Dairy strains
of propionibacteria are autolytic under environmental conditions found in
cheese and degrade peptides and amino acids that are present in the cheese. and
The dairy species offer an interesting
opportunity as novel probiotic organisms with the most obvious advantage being
that they are considered safe for ingestion.
1.2.19 Pediococcus spp.
(Pediococcus pentosaceus, P. acidilactici)
Pediococci
compromise a group of bacteria that are of economic importance in the brewing
and food industries. Several species and strains of pediococci have been used
as starter cultures in the fermentation of vegetables, meats, sausage products,
fermented milks and associated with the development of flavor in Cheddar and
other related cheese varieties. Some strains form capsular material that causes
beer to become ropy and viscous.
They are catalase negative and exhibit a
homolactic type of fermentation and produce optically inactive lactic acid i.e.
a mixture of the L(+) and D(-) type. They generally appear in tetrads.
1.2.20 Pseudomonas
fluorescens
Pseudomonas
fluorescens is a common
Gram-negative, rod-shaped, motile bacterium. The organism is psychrotrophic in
nature and grows at refrigeration temperature (7°C). It has an extremely
versatile metabolism, and can be found in the soil and in water. It is an
obligate aerobe, but certain strains are capable of using nitrate instead of
oxygen as a final electron acceptor during cellular respiration. Optimal
temperature for growth of Pseudomonas fluorescens is 25-30 °C.
It tests positive for the oxidase. Pseudomonas fluorescens is
also a nonsaccharolytic organism. Heat-stable lipases and proteases are
produced by Pseudomonas fluorescens and other similar
pseudomonads. These enzymes cause milk to spoil, by causing bitterness, casein
breakdown, and ropiness due to the production of slime and coagulation of
proteins.
1.2.21 Pseudomans
aeruginosa
It is a Gram-negative, aerobic, rod-shaped
bacterium with unipolar motility. An opportunistic human pathogen, P.
aeruginosa is also an opportunistic pathogen of plants. P.
aeruginosa is the type species of the genus Pseudomonas (Migula).
Gram-stained Pseudomonas aeruginosa bacteria (pink-red rods)
secretes a variety of pigments, including pyocyanin (blue-green), pyoverdine
(yellow-green and fluorescent), and pyorubin (red-brown). P. aeruginosa is
often preliminarily identified by its fluroscence and grape-like or tortilla-like
odor in vitro. Definitive clinical identification of P.
aeruginosa often includes identifying the production of pyocyanin and
fluorescein, as well as its ability to grow at 42°C. P. aeruginosa is
capable of growth in diesel and jet fuel, where it is known as a
hydrocarbon-using microorganism (or "HUM bug"), causing microbial
corrosion. P. aeruginosa is considered by many as a
facultative anaerobe
1.2.22 Salmonella (S.
typhimurium, S. typhi, S.enteritidis)
Salmonella spp. have been reported to be a leading cause of foodborne
illnesses in humans. Foodborne salmonellosis scores over all other foodborne
bacterial illnesses in humans. Enteric fever is a serious human disease
associated with typhoid and paratyphoid strains. SalmonellaEnterobacteriaceae.
The optimum growth temperature is 37-45 °C. The organism can also grow at about
7°C in foods. I t ferments carbohydrates with its production of acid and gas.
Salmonella are oxidase negative, catalase positive and grow on citrate as a
sole carbon source and produce H2S. Some Salmonella strains
can grow at higher temperatures (54 °C) while others exhibit psychrotrophic
properties. The organism has the ability to grow at pH values ranging from 4.5
to 9.5, with an optimum pH growth at 6.5 to 7.5. spp. are facultatively
anaerobic, small Gram-negative, non spore forming, rod-shaped (2-4 m m)
bacteria belonging to the family
Milk, meat and poultry are principle vehicles of
human foodborne salmonellosis. Ingestion of only a few salmonella cells can be
infectious. Low levels of salmonellae in a finished food products may,
therefore, be of serious public health consequence.
1.2.23 Serratia
Serratia is a genus of Gram-negative, facultatively anaerobic, rod-shaped
bacteria of the Enterobacteriaceae family. The most common
species in the genus, S. marcescens, is normally the only pathogen
and usually causes nosocomial infections. However, rare strains of S.
plymuthica, S. liquefaciens, S. rubidaea, and S.
odoriferae have caused diseases through infection. Members of this
genus produce characteristic red pigment, prodigiosin.
1.2.24 Streptococcus
thermophilus
The
only streptococcus species that is associated with food
technology is S. thermophilus which is used in the
manufacture of yoghurt (in co culture with L. bulgaricus
and Dahi).
S.
thermophilus is a Gram
positive facultative anaerobe and belongs to the family Streptococcaceae.
It is catalase negative organism that is non-motile, non-spore forming and
homofermentative and occurs in pairs to long chains. The spherical to avoid
cells are with a diameter in the range of 0.7 to 0.9 µm. The optimum
temperature for the growth of this organism is between 39°C to 45°C, although
most species in the genus are able to grow at temperature ranging from 45-60°C.
They do not grow at temperature below 20°C, but they can survive at
65°C for 30 min. They ferment sugars with L (+) lactic acid as the major
end product and produce around 0.6 to 0.8% lactic acid. They are able to grow
in broth with 2.5% NaCl but fail to grow in 6.5% NaCl at pH 9.6 or in milk with
0.1% methylene blue (Bergey’s Manual 1994). It is also classified as lactic
acid bacteria (LAB). It is a very versatile organism. S. thermophilus has
properties that make it one of the commercially most important lactic acid
organism. S. thermophilus is used along with Lactobacillus spp.,
as a starter culture to manufacture several important fermented dairy foods
including yoghurt and mozzarella cheese.
Though the natural habitat of S.
thermophilus is yet to be established, most strains have been isolated
from milk environments.
1.2.25 Staphylococcus
aureus
Staphylococcus
aureus is commonly associated
with humans. It is a Gram-positive catalase-positive coccus. Staphylococcus
aureus is the common cause of foodborne gastroenteritis known as
staphylococcal food poisoning. Staphylococcal gastroenteritis is caused by the
ingestion of food that contains one or more enterotoxin which are produced by
some strains of S. aureus.
Although
enterotoxin production is believed generally to be associated with coagulase
and thermo nuclease producing S. aureus strains, many species
of Staphylococcus that produce neither coagulase nor TNase are
also known to produce enterotoxin.
The main
reservoir of S. aureus is the nasal cavity of human beings
from where they find their way to the skin and wounds. Mastitis in animals due
to S. aureus is quite common and from the infected udder the
organism finds its way to the milk.
The organism can grow well in NaCl
concentrations of 7 to 10%. Though the optimum growth temperature of the
organism is 37 °C , some strains can grow at a temperature as low as
6.7 °C. The organism can grow to water activity as low as 0.86.
1.2.26 Shigella
Bacillary
dysentery, or shigellosis, is caused by Shigella species. Shigella is
a member of the family Enterobacteriaceae. The growth temperature
varies from 10 to 48 °C. Shigella2S. Shigella does
not usually survive well in low pH foods. Shigella is
sensitive to ionizing radiations. species are non-motile, oxidase negative
produce acid only from sugars; do not grow on citrate as sole carbon source, do
not grow on KCN agar, and unlike Salmonellae do not produce H
Shigellosis is an important disease in developed
and developing countries. Disease is caused by ingestion of contaminated foods,
and in some instances it subsequently leads to rapid dissemination through
contaminated feces from infected individuals. The infective dose may be as low
as 100 cells. Contamination of foods usually does not occur at the processing
plant but rather through an infected food handler. Humans are the natural
reservoir of Shigella. The organism is spread through the fecal-oral route.
1.2.27 Vibrio
Vibrio
cholerae and V.
parahaemolyticus are the two important species of the genus Vibrio. Vibrio
cholerae O1 causes cholera, one of the few food borne illnesses with
epidemic and pandemic potential. Vibrio cholerae are
Gram-negative straight or curved rods and belong to the family Vibrionaceae.
Important distinctions within the species are made on the basis of productions
of cholera enterotoxin (CT) and serogroup.
Vibrio cholerae is part of the normal
free living bacterial flora in estuarine areas. Amongst the many different
enrichment broths described for the isolation of vibrios alkaline peptone water
is the most commonly used. Though V. parahaemolyticus can grow
in the presence of 1-8% NaCl, the best growth occurs in the salt concentration
2 to 4%.
1.2.28 Yersinia
Yersinia
enterocolitica and Yersinia
pestis are the two important human pathogens while Y.
enterocolitica causes food borne gastroenteritis, Y. pestis is
an agent of human plague. Y. enterocolitica also known as
newly emerging human pathogen is a heterogeneous species that is divisible into
a large number of subgroups.
Y.
enterocolitica is unusual because
it can grow at temperatures below 4 °C. The generations time at the
28-30 °C (Optimum growth temperature) is almost 34 min. It also
survives in frozen foods. It grows better in processed foods such as
pasteurized milk, vacuum packed meat, boiled eggs, boiled fish, and cottage
cheese.
Both the
species can grow over a pH range of 4 to 10 (optimum pH is 7.6) and tolerate
alkaline environment well. They can motile at a temperature < 30 °C.
However, both these organisms are susceptible to pasteurization, ionizing and
ultraviolet (UV) irradiation. The organism can also tolerate upto 5% NaCl.
Infections with Yersinia species
are due to transmittance of the organism from animals to humans. The organism
is frequently present in pork, lamb, poultry and dairy products.
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