Lecture 4 - Food microbiology

Question 1

Biological hazards in food can be:

Answer 1

prions
viruses
bacteria
mold and yeasts protozoa

Question 2

Bacteria (Bacteria; Greece word bacteria ‘stick’) are the smallest unicellular prokaryotic organisms which are individually able to

Answer 2

grow and multiply.

Question 3

First bacteria probably occurred ~ when?

Answer 3

3.5 billion years ago and it is believed that they were first living organisms in earth.

Question 4

On and inside of human body the number of bacteria exceeds the number of body cells by

Answer 4

at least ten times.

Question 5

The bacteria that can cause infection (or intoxication) are termed

Answer 5

pathogenic bacteria.

Question 6

Most pathogenic bacteria are able to produce

Answer 6

toxins, which usually cause tissue
damage.

Bacterial toxins are generally protein toxins.

Question 7

Prokaryotic cells are simple in structure, with no

Answer 7

recognizable organelles.

Question 8

Bacteria have an outer cell wall that gives them shape. Just under the rigid cell wall is

Answer 8

the more fluid cell membrane.

Question 9

Bacteria According to their shape: (3)

Answer 9

cocci
rods/bacilli
spiral

Question 10

The nature of the cell wall defines bacteria into

Answer 10

Gram-negative and Gram-positive which can be identified via Gram
staining.

Gram-positive bacteria have thick peptidoglycan layer.

Question 11

define psychrotroph
define psychrophile

Answer 11

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

Question 12

define mesophile and thermophile

Answer 12

mesophile: An organism, especially a microorganism, that lives and thrives at moderate temperatures. thermophile: An organism that lives and thrives at relatively high temperatures.

Question 13

How do bacteria survive in low and high temps?

Answer 13

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.

Question 14

Pathogenic bacteria will grow the best at what temperature

Answer 14

human body temperature 37 ºC.

Most are growing at temperature range 15-45 ºC.

Question 15

Vegetative bacteria are killed
at temperatures …?

Answer 15

more than 60 ºC.

• time factor depends on organism

Boiling destroys vegetative bacteria but not spores.

Question 16

Refrigerators must be regulated to

Answer 16

5 ºC.

Some bacteria e.g. Listeria monocytogenes are able to grow at
refrigerated temperatures.

Question 17

According to the impact of human health the microorganisms can be divided into four groups:

Answer 17

1. hazardous to human health – pathogens including foodborne
   pathogens
2. non-pathogenic or conditionally pathogenic bacteria which can be named also as indicator organisms
3. food spoilage microflora
4. beneficial microorganisms to human health

Question 18

Detection of indicator micro-organisms in food may be used for:

Answer 18

evaluate the efficiency of heat treatment

control the hygiene status

etc.

Question 19

examples of Indicator (micro)organism

Answer 19

Escherichia coli, Coliforms and Enterobacteriaceae

Question 20

Common foodborne genera of the Family Enterobacteriaceae include:

Answer 20

Citrobacter, Enterobacter, Erwinia, Escherichia, Hafnia, Klebsiella, Proteus, Providencia, Salmonella, Serratia, Shigella, and Yersinia

Question 21

The Enterobacteriaceae have been used for years in Europe as

Answer 21

indicators of food quality and indices of food
safety.

Question 22

Process hygiene indicators, Coliforms

The coliform group is defined on the basis of

Answer 22

biochemical reactions, not genetic relationships, and thus the
term “coliform” has no taxonomic validity.

Question 23

Examples of Process hygiene indicators, Coliforms.

Answer 23

Mainly representatives of the Genus Escherichia, Enterobacter, Klebsiella ja Citrobacter.

Question 24

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

Answer 24

a very specific indicator of fecal contamination.

Question 25

The source of coliforms in RTE-foods after thermal processing is usually

Answer 25

the processing environment, resulting
from inadequate sanitation procedures and/or temperature control.

Question 26

Escherichia coli is

Answer 26

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.

Question 27

the definitive organism for the demonstration of fecal contamination of water and food is

Answer 27

E.coli

The failure to detect E. coli in a food, however, does not assure the absence of enteric pathogens.

Question 28

Typical growth rate of bacteria at optimal conditions

Answer 28

20 min

Question 29

Explain lag-phase. (Microbial growth phase)

Answer 29

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.

Question 30

Explain Logarithmic growth phase [exponential (log) phase]

Answer 30

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

Question 31

The rate of exponential growth of a bacterial culture is expressed as

Answer 31

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.

Question 32

Explain Stationary Microbial growth phase

Answer 32

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.

Question 33

Explain microbial death phase.

Answer 33

During the death phase, the number of viable cells decreases geometrically (exponentially), essentially the reverse of growth during the log phase.

Question 34

When growing exponentially by binary fission, the increase in a bacterial population is by

Answer 34

geometric progression.

Question 35

Exotoxins are produced when

Answer 35

bacteria grow in food so microbial growth

(many are heat resistant, short incubation period).

Question 36

Endotoxins are found where

Answer 36

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.

Question 37

Enterotoxins are

Answer 37

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

Question 38

Spores are

Answer 38

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.

Question 39

What is a vegetative bacterial cell?

Answer 39

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.

Question 40

Name two bacterial genuses that are spore forming.

Answer 40

Bacillus
Clostridium

Question 41

Destruction of bacteria in food, 5 methods using heat

Answer 41

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)

Question 42

Destruction of bacteria in food, 3 non-heat based methods

Answer 42

CHEMICALS
e.g. chlorine in water

IRRADIATION
Only spices licensed for irradiation in the EU

UV LIGHT
Water/shellfish purification.

Question 43

Classification of viruses is based on characteristics such as (3)

Answer 43

genetic material (RNA or DNA),
viral particles symmetry and
presence or absence of the shell.

Question 44

Structurally there are 2 main types of viruses:

Answer 44

with a lipid envelope around the capsid and without lipid envelope around the capsid (naked).

Question 45

Foodborne virus reservoirs: (2)

Answer 45

– drinking water

– different foods like oysters, mussels, frozen foods (e.g. berries), dairy foods, fruits and vegetables etc.

Question 46

HAV

Answer 46

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).

Question 47

Fungi which are causing human diseases fall into two clearly morphologically different form:

Answer 47

– 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.

Question 48

What kinds of infections are caused by microfungi?

Answer 48

superficial mycoses (e.g. candida albicans) and inner mycoses (rare, usually life threatening)

Question 49

The most important hazards regarding to molds in foods are their produced
mycotoxins, especially..?

Answer 49

aflatoxins. Aflatoxins are carcinogenic, mutagenic, teratogenic and liver
toxic.

for example aflatoxins produced by Aspergillus flavus and Aspergillus parasiticus
species.

Question 50

Aflatoxins are divided into groups: (4)

Answer 50

B1, B2, G1 and G2

– carcinogenic properties are in B1 and G1 group.

Question 51

The most hazardous aflatoxin is

Answer 51

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.

Question 52

Most important protozoa to note: (4)

Answer 52

Cryptosporidium spp.
Giardia lamblia
Entamoeba histolytica
(colon infection causing amoeba; causing diarrhea and spread via contaminated food and drinking water)
Toxoplasma gondii (CNS)

Question 53

Stability of Giardia and Cryptosporidium

Answer 53

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.

Question 54

Human cryptosporidiosis

Answer 54

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.

Question 55

Toxoplasmosis is

Answer 55

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.

Question 56

Lambliosis/giardiasis is

Answer 56

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.