Lecture 4 - Food microbiology Flashcards
Biological hazards in food can be:
prions
viruses
bacteria
mold and yeasts protozoa
Bacteria (Bacteria; Greece word bacteria ‘stick’) are the smallest unicellular prokaryotic organisms which are individually able to
grow and multiply.
First bacteria probably occurred ~ when?
3.5 billion years ago and it is believed that they were first living organisms in earth.
On and inside of human body the number of bacteria exceeds the number of body cells by
at least ten times.
The bacteria that can cause infection (or intoxication) are termed
pathogenic bacteria.
Most pathogenic bacteria are able to produce
toxins, which usually cause tissue
damage.
Bacterial toxins are generally protein toxins.
Prokaryotic cells are simple in structure, with no
recognizable organelles.
Bacteria have an outer cell wall that gives them shape. Just under the rigid cell wall is
the more fluid cell membrane.
Bacteria According to their shape: (3)
cocci
rods/bacilli
spiral
The nature of the cell wall defines bacteria into
Gram-negative and Gram-positive which can be identified via Gram
staining.
Gram-positive bacteria have thick peptidoglycan layer.
define psychrotroph
define psychrophile
Psychrophilic and psychrotrophic microorganisms have the ability to grow at 0 degree C.
Psychrotrophic microorganisms have a maximum temperature for growth above 20 degrees C and are widespread in natural environments and in foods.
min. growth temp of 10’C
define mesophile and thermophile
mesophile: An organism, especially a microorganism, that lives and thrives at moderate temperatures. thermophile: An organism that lives and thrives at relatively high temperatures.
How do bacteria survive in low and high temps?
higher amount of unsaturated fatty acids maintains plasma membrane in a liquid and mobile state at low temperatures.
high content of saturated fatty acids maintains plasma membrane abnormally stable at high temperatures.
the cell proteins and enzymes are unusually heat stable.
Pathogenic bacteria will grow the best at what temperature
human body temperature 37 ºC.
Most are growing at temperature range 15-45 ºC.
Vegetative bacteria are killed
at temperatures …?
more than 60 ºC.
- time factor depends on organism
Boiling destroys vegetative bacteria but not spores.
Refrigerators must be regulated to
5 ºC.
Some bacteria e.g. Listeria monocytogenes are able to grow at
refrigerated temperatures.
According to the impact of human health the microorganisms can be divided into four groups:
- hazardous to human health – pathogens including foodborne
pathogens - non-pathogenic or conditionally pathogenic bacteria which can be named also as indicator organisms
- food spoilage microflora
- beneficial microorganisms to human health
Detection of indicator micro-organisms in food may be used for:
evaluate the efficiency of heat treatment
control the hygiene status
etc.
examples of Indicator (micro)organism
Escherichia coli, Coliforms and Enterobacteriaceae
Common foodborne genera of the Family Enterobacteriaceae include:
Citrobacter, Enterobacter, Erwinia, Escherichia, Hafnia, Klebsiella, Proteus, Providencia, Salmonella, Serratia, Shigella, and Yersinia
The Enterobacteriaceae have been used for years in Europe as
indicators of food quality and indices of food
safety.
Process hygiene indicators, Coliforms
The coliform group is defined on the basis of
biochemical reactions, not genetic relationships, and thus the
term “coliform” has no taxonomic validity.
Examples of Process hygiene indicators, Coliforms.
Mainly representatives of the Genus Escherichia, Enterobacter, Klebsiella ja Citrobacter.
Coliforms are ubiquitous in nature, therefore a number of factors should be considered when testing for a particular indicator organism such as the native microbiota of the food, the extent to which the food has been processed, and the effect that processing would be expected to have on the indicator organisms.
Because of the widespread nature of these bacteria, the Coliforms group of bacteria is not
a very specific indicator of fecal contamination.
The source of coliforms in RTE-foods after thermal processing is usually
the processing environment, resulting
from inadequate sanitation procedures and/or temperature control.
Escherichia coli is
A gram negative rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms.
It is part of the normal microbiota of the human and animal gut.
Most E. coli strains are harmless, but some, such as serotype O157:H7 can cause serious food poisoning in humans.
the definitive organism for the demonstration of fecal contamination of water and food is
E.coli
The failure to detect E. coli in a food, however, does not assure the absence of enteric pathogens.
Typical growth rate of bacteria at optimal conditions
20 min
Explain lag-phase. (Microbial growth phase)
microbes adapt to new environmental conditions; production of different
growth factors.
although there is no apparent cell division occurring, the cells may be growing in volume or mass, synthesizing enzymes, proteins, RNA, etc., and increasing in metabolic activity.
at the end of this phase cells will divide into two.
Explain Logarithmic growth phase [exponential (log) phase]
the exponential phase of growth is a
pattern of balanced growth wherein all the cells are dividing regularly by binary fission, and are growing by geometric progression.
The cells divide at a constant rate depending upon the composition of the growth medium and the conditions of incubation
The rate of exponential growth of a bacterial culture is expressed as
generation time, also the doubling time of the bacterial population.
Generation times for bacteria vary from about 12 minutes to 24 hours or more.
The generation time for E. coli in the laboratory is 15-20 minutes, but in the intestinal tract, the coliform’s generation
time is estimated to be 12-24 hours.
Explain Stationary Microbial growth phase
Stationary phase results from a situation in which growth rate and death rate are equal.
The number of new cells created is limited by the growth factor and as a result the rate of cell growth matches the rate of cell death.
The result is a “smooth,” horizontal linear part of the curve during the stationary phase.
Explain microbial death phase.
During the death phase, the number of viable cells decreases geometrically (exponentially), essentially the reverse of growth during the log phase.
When growing exponentially by binary fission, the increase in a bacterial population is by
geometric progression.
Exotoxins are produced when
bacteria grow in food so microbial growth
(many are heat resistant, short incubation period).
Endotoxins are found where
in the composition of the cell and get into the environment after cell death (cell lysis; after the
bacterial cell wall has collapsed)
also released when spores are formed.
E.g. Endotoxin = lipopolysaccharide complex associated with the
outer membrane of Gram-negative pathogens such as Escherichia coli, Salmonella, Shigella, Pseudomonas, Neisseria etc.
Enterotoxins are
protein toxins which are released by microorganisms in intestines; are mostly cytotoxic; cause destruction of intestinal epithelium cells.
Enterotoxins can be produced by Escherichia coli O157:H7, Clostridium perfringens, Vibrio cholerae, Staphylococcus aureus, Shigella dysenteriae (Shiga-toxin), Yersinia enterocolitica
Spores are
a Resistant resting phase of bacteria. they do not multiply.
Survive:
* High temperatures, e.g. boiling for up to 5 hours
* Disinfectants
* Dehydration
Examples: Clostridium and Bacillus.
What is a vegetative bacterial cell?
A cell of a bacterium or unicellular algae that grows actively and forms endospores is known as vegetative cell. The spores are dehydrated and metabolically inactive.
Name two bacterial genuses that are spore forming.
Bacillus
Clostridium
Destruction of bacteria in food, 5 methods using heat
Pasteurization
(destroys pathogens and some spoilage bacteria)
Sterilization
(destroys all bacteria, spores and toxins)
UHT
(ultra heat treated)
Canning
(commercially sterile, designed to destroy Clostridium botulinum spores)
Effective cooking (core temperature at least 75°C)
Destruction of bacteria in food, 3 non-heat based methods
CHEMICALS
e.g. chlorine in water
IRRADIATION
Only spices licensed for irradiation in the EU
UV LIGHT
Water/shellfish purification.
Classification of viruses is based on characteristics such as (3)
genetic material (RNA or DNA),
viral particles symmetry and
presence or absence of the shell.
Structurally there are 2 main types of viruses:
with a lipid envelope around the capsid and without lipid envelope around the capsid (naked).
Foodborne virus reservoirs: (2)
– drinking water
– different foods like oysters, mussels, frozen foods (e.g. berries), dairy foods, fruits and vegetables etc.
HAV
Hepatitis A Virus, enterovirus group of family Picornaviridae. distributed worldwide.
more common in regions of the world with poor sanitation and not enough safe water
Primarily transmitted by fecal-oral route (person-to-person spread).
Fungi which are causing human diseases fall into two clearly morphologically different form:
– Yeasts
grow as oval or spherical single cells like bacteria and multiply by division or by budding.
– Fungi with mycelium
also known as molds; consist of long, branched hypha which form mycelium. multiply asexually by forming spores.
What kinds of infections are caused by microfungi?
superficial mycoses (e.g. candida albicans) and inner mycoses (rare, usually life threatening)
The most important hazards regarding to molds in foods are their produced
mycotoxins, especially..?
aflatoxins. Aflatoxins are carcinogenic, mutagenic, teratogenic and liver
toxic.
for example aflatoxins produced by Aspergillus flavus and Aspergillus parasiticus
species.
Aflatoxins are divided into groups: (4)
B1, B2, G1 and G2
– carcinogenic properties are in B1 and G1 group.
The most hazardous aflatoxin is
B1, because of very high biological activity and is active in many biochemical reactions of the body.
B1 damages liver DNA structure and causes liver cancer.
Most important protozoa to note: (4)
Cryptosporidium spp.
Giardia lamblia
Entamoeba histolytica
(colon infection causing amoeba; causing diarrhea and spread via contaminated food and drinking water)
Toxoplasma gondii (CNS)
Stability of Giardia and Cryptosporidium
Resistant in water environments, both to temperature and other factors
Resistant to water treatment processes, chlorine tolerance (50-80 mg/l).
Infectious dose is low – 10 to 100 cysts or oocysts is sufficient to cause infection.
Human cryptosporidiosis
Incubation period: 1-12 days
For healthy person:
– asymptomatic or
– acute self limiting gastroenteritis
up to 13 days but 10% of patients need fluid therapy.
For immunocompromised people it can be life
threatening disease:
* lung or tracheal cryptosporidiosis
* mild fever (not more than 38 ºC)
* serious intestinal disorders.
Toxoplasmosis is
a parasitic disease caused by Toxoplasma gondii.
Toxoplasmosis is usually spread by eating poorly cooked food that contains the cysts, exposure to infected cat feces, and from a mother to a child during pregnancy if the mother becomes infected.
main host is the cat
Often the symptoms do not occur at all and man can be the carrier of parasite for many years.
Lambliosis/giardiasis is
a zoonotic parasitic disease caused by the flagellate protozoan Giardia lamblia (also
called Giardia intestinalis and Giardia duodenalis).
The giardia organism inhabits the digestive tract of a wide variety of domestic and wild animal species, as well as humans.
It is the most common pathogenic parasitic infection in humans worldwide. Infection occurs mainly via contaminated environment or domestic animals.
