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1. GROUPS OF BACTERIA IMPORTANCE IN FOOD MICROBIOLOGY
Presented by -
Tamanna Naznin
181706
APPT

2. Introduction
• Bacteria important in food microbiology may be divided into
groups according to the product of fermentation, e.g. lactic
acid bacteria, acetic acid bacteria, propionic acid bacteria.
• The micro-organisms occurring on and/or in foods are from a
practical point of view divided into three groups: molds, yeast
and bacteria. Molds are generally concerned in the spoilage of
foods; their use in the food industry is limited (e.g. mold
ripened cheese).

3. • Yeasts are the most widely used micro-organisms in the food
industry due to their ability to ferment sugars to ethanol and
carbon-dioxide. Some types of yeast, such as baker’s yeasts
are grown industrially, and some may be used as protein
sources, mainly in animal feed.
• Their systematic classification is based primarily on
morphological and physiological properties (e.g. aerobic and
anaerobic bacteria, gas forming bacteria, etc.). Lactic acid
bacteria are widely used in the dairy industry, and acetic acid
bacteria in vinegar production. Many bacteria are known as
micro-organisms that cause spoilage and some are pathogens
(e.g. salmonellae, staphylococci,etc.

4. Thermophilic bacteria or Thermophile
• This bacteria with an optimal temperature at least 45°C but
usually 55 °C or above are important in foods held at high
temperatures.
• Thermophilic flat sour spoilage of low acid canned foods is
caused by B.stearothermophilus.
• Gaseous Thermophilic spoilage of canned foods is a result of
growth by C.thermosaccharolyticum.

5. Thermoduric bacteria
• Thermoduric bacteria is usually defined as those which can
survive a heat treatment such as pasteurization.
• Bacillus species, micrococci and enterococci can survive
pasteurization of liquid eggs.
• Bacteria in the genera Clostridium, Bacillus, Micrococcus ,
Streptococcus, Lactobacillus and Micro-bacterium are
frequently encountered in foods.

6. • Occasionally molds such as Byssochlamy fulva and even
Aspergillus and Penicillum are Thermoduric.
• Some Thermoduric bacteria such as Bacillus and enterococci
can also be psychotropic.
• In the milk, where higher pasteurization and longer
refrigeration times are significant, these heat resistant, or
Thermoduric, Psychrotrops often can be found.

7. Psychrophilic bacteria
• Psychrophilic bacteria are defined as cold-loving bacteria.
Specifically, their cardinal temperatures are 20 °C for maximal
growth, 15 °C or lower for optimal growth, and 0 °C or lower
for minimum growth and this definition is accepted by most
microbiologists. The old definition of psychrophiles applied to
those organisms that produced a visible colony in 1 week at
0 °C.
• From an ecological standpoint, psychrotrophs and
psychrophiles are both found in cold environments, but
psychrophiles are not found at temperatures higher than
20 °C.

8. • This temperature was selected as the maximum temperature
for growth based on the fact that laboratory temperatures in
the United States are approximately 21 or 22 °C.
• Microorganisms capable of growing at 5 °C or lower are
psychrotrophs, regardless of the optimum temperature for
growth. The psychrotrophs are cold-tolerant bacteria, but
their maximal growth temperature ranges above 20 °C and in
many cases their optimal growth temperature is also above
20 °C.

9. • A better term for these organisms that withstand cold
temperatures is psychrotolerant. However, because of
common usage this term should be retained. For example,
there are a few texts employing the term “psychrotrophs” in
their title and this term is widely used by industry (mainly
dairy and food), e.g., the normal souring of milk is due to
psychrotrophs.

10. Psychrotrophic bacteria
• Psychrotrophic microbes are able to grow at temperatures
below 7 °C (44.6 °F), but have better growth rates at higher
temperatures. Psychrotrophic bacteria and fungi are able to
grow at refrigeration temperatures, and can be responsible
for food spoilage. They provide an estimation of the product's
shelf life, but also they can be found in soils, in surface and
deep sea waters , in Antarctic ecosystems , and in foods.
• Psychrotrophic bacteria are of particular concern to the dairy
industry. Most are killed by pasteurization; however, they can
be present in milk as post-pasteurization contaminants due to
less than adequate sanitation practices.

11. • These bacteria are able to grow at commercial refrigeration
temperatures. Unlike psychrophiles, psychrotrophs do nor
have their optimal temperature for growth at refrigeration
temperatures; rather their optimal is usually between 25 °C
and 30 °C.
• Most of the bacteria responsible for the loss of quality in
nonsterile refrigerated foods, excluding sea-foods are
psychrotrophs.

12. • Psychrotrophs bacteria are found chiefly in genera
Pseudomonas, Flavobacterium, Achromobacter, and
Alcaligenes, although Micrococcus, Lactobacillus,
Enterobacter, Arthrobacter, and other genera may contain
Psychrotrophic species.
• Additionally, various yeasts and molds are able to grow at
refrigeration temperature.

13. Halophilic Bacteria or Halophiles
• Truly Halophilic bacteria require certain minimal
concentrations of dissolved sodium chloride for growth.
• Those bacteria including Pseudomonas, Moraxella,
Flavobacterium, Acinetobacter, and Vibrio species, which
grow best in media with 0.5-3.0 % salt, are considered slightly
Halophilic.
• Bacteria which are isolated from foods such as fish, brined
meat, and some salted vegetables and which grow best I
media with 3.0 to 15% salt are referred to as moderate
Halophiles.

14. • Occasionally in heavily brined foods, 15-30% salt, extremely
Halophiles such Halo-bacterium and Halococcus species can
be isolated.
• Frequently they are also pigmented pink or red.
• Other bacteria are salt tolerant; i.e., halotolarant bacteria can
grow with or without salt. Usually they are capable of growing
in foods containing 5% salt or more; they include some
Bacillus, Micrococcus, Corynebacterium, Streptococcus, and
clostridium species.
• Other Halophilic or halo-tolerant bacteria important in foods
are found in the genera Sarcina,Pseudomonas, Pediococcus,
and Alcaligenes.

15. Osmophilic or Saccharophilic Bacteria
• The frequently encountered Osmophilic microorganisms in foods are
various species of yeasts.
• Osmophilic bacteria are those which grow in high concentrations of sugar;
however, most bacteria called Osmophiles are merely sugar-tolerant, e.g.,
species of Leuconostoc.
• Such bacteria are found in the genera Bacillus, Micrococcus, Vibrio,
Actinobacter, and Moraxella.

16. Gas forming Bacteria
• Many kind of bacteria produce such small amounts of gas and
yield it so slowly that it ordinarily is not detected.
• This sometimes is true of the hetero-fermentative lactics,
although under other conditions gas evolution is evident.
• Among the genera that contain gas-forming bacteria are
Leuconostoc, Lactobacillus (heterofermentative),
Propionibacterium, Escherichia, Enterobacter, Proteus,
Bacillus(the Aero-bacilli),and clostridium.
• Bacteria of the first three genera produce only CO2, and those
of the other genera yield both CO2 and H2.

17. Coliform and Fecal Coliform Group
• Coliforms are short rods that are defined as aerobic and
facultative anaerobic, Gram negative, non-spore-forming
bacteria which ferment lactose with gas formation.
• The leading species of Coliform bacteria are Escherichia coli
and Enterobacter aerogenes ; however, as many as 20 species
may conform to these criteria, including species of other
Enterobacteriaceae, and even Aeromonas species.
• The fecal Coliform groups includes Coliform capable of growth
at an elevated temperature (44.5 or 45 °C).

18. • The original purpose of elevated incubation tests was to
differentiate Coliforms of fecal origin from those of non fecal
origin.
• The designation of “Fecal Coliform” or “Coliform” is not
taxonomically valid; rather, the terms refer to the groups of
bacteria that can grow under specific tests conditions.