FOOD MICROBIOLOGY

By: Kimberly Le

Background Information

Microbiology: the study of microorganisms/

microbes which are a diverse group of simple

life-forms including bacteria, archaea, algae,

fungi, protozoa's, and virus.

Food Microbiology: the study of

microorganisms that inhibit, create, or

contaminate food.

◦ The foods we eat are rarely sterile, they

carry microbes that interact with food.

Microbes in food may display their

presence in several ways:

◦ They can cause food spoilage

◦ They can cause foodborne illnesses

◦ They can transform food’s properties in a

beneficial way (food fermentation)

Antonie van Leeuwenhoek,

first scientist to observe and

describe microorganisms.

Also designed the first

microscope.

Louis Pasteur, father of food

microbiology. Created

pasteurization (prevent food

spoilage) and developed

vaccines.

This Photo by Unknown Author

This Photo by Unknown Author

This Photo by Unknown Author

Food Spoilage

According to the Food and

Agriculture Organization

(FAO), an estimated 1/3 of

the food produced globally is

either lost or wasted,

amounting to 1.3 billion tons

per year.

◦ According to the Oxford dictionary, to spoil is to

“deprive of good or effective qualities”.

◦ Food characteristics change when spoiled and is no

longer acceptable for consumption. Spoiled does not

always come from microbiological in origin but the

majority is caused by microbial activity. (Ex: insect

damage, drying out, discoloration, staling or rancidity.)

◦ Visible microbial growth on surface of food product

(slime or colonies), loss of texture, or chemical

production of microbial metabolism (gas,

pigmentation, off-odors, and flavors) are indicators.

◦ Spoilage is an indicator of quality. An individual may

view it as unacceptable while the other does not due to

social or cultural views. (Ex: matured cheese or game

birds)

Foodborne Illnesses ◦ The World Health Organization (WHO) defines foodborne disease as “Any disease of an infectious or toxic nature caused by, or

thought to be caused by, the consumption of food or water”.

◦ Foodborne illnesses (also known as food poisoning) are commonly caused by different types of bacteria and symptoms vary

among different types.

◦ Most common symptoms include nausea, vomiting, stomach cramps, and diarrhea. Maybe severe or sometimes life threatening.

◦ Five common germs that cause illnesses in the United States according to CDC:

◦ Norovirus

◦ Salmonella

◦ Clostridium perfringens

◦ Campylobacter

◦ Staphylococcus aureus

◦ Three that are considered zero tolerance (potentially dangerous at any level when ingested, does not cause illness but are more

likely to lead to hospitalization):

◦ Salmonella

◦ E.coli

◦ Listeria monocytogenes

Norovirus (NoV)

◦ There are at least 33 different norovirus genotypes but

there are only 3 genogroups that infect humans.

◦ Very contagious virus that causes projectile and explosive

vomiting, followed by diarrhea among people of all ages.

A person usually develops symptoms 12 to 48 hours after

exposure and the symptoms may last 1 to 3 days.

◦ Able to contract the virus by direct contact with an

infected person, consuming contaminated food or water,

or touching a contaminated surface.

◦ Common settings of outbreaks includes healthcare

facilities, restaurants, schools, childcare centers, and

cruise ships.

◦ Sources:

◦ Contaminated water

◦ Seafood

◦ Meat

◦ Fruit and vegetables

This Photo by Unknown Author is licensed under CC BY-SA-NC

According to the CDC,

there are about 2,500

reports of Norovirus

outbreaks each year in the

US. Most common from

November to April.

Leading

cause of

foodborne

illness in

the US

Salmonella◦ A motile, non-spore forming, gram negative, rod-

shape bacterium.

◦ Divided into two species: S. enterica and S. bongoriwith subspecies but is further subdivided by serotypes.

◦ Can cause two types of illnesses, depending on serotype: nontyphoidal and typhoid.

◦ Typhoid (S. Typhi and S. Paratyphi A. which is found only in humans)

◦ Duration is longer than nontyphoidal

◦ Most severe and has a higher rate of mortality

◦ Usually associated with contaminated drinking water

◦ Nontyphoidal (caused by serotypes other than those caused by typhoid)

◦ Sources:

◦ Contaminated water

◦ Food sources (Ex: meat, poultry, eggs, milk, fish, produce, etc.)

◦ Cross contamination

This Photo by Unknown Author is licensed under CC BY-ND

This Photo by Unknown Author is licensed under CC BY-SA

Growth of Salmonella on Xylose

Lysine Deoxycholate agar (XLD)

plate.

Salmonella bacterium

This Photo by Unknown Author is licensed under CC BY-SA

Bacteria was

discovered by an

American

scientist, Daniel

E. Salmon in

1885.

Salmonella is more

common in the summer

due to warmer weather

and unrefrigerated foods

creating an ideal

condition for the bacteria

to grow.

Clostridium perfringens

◦ Anaerobic (but aerotolerant), gram positive, spore-forming rod

that produces an enterotoxin.

◦ Non-pathogenic C. perfringens is distributed in the environment

and is found in the intestines of humans.

◦ Can take on 2 forms in foodborne illnesses:

◦ Gastroenteritis form

◦ Common and self limiting

◦ Depending on strain, can cause more severe damage

◦ Enteritis necroticans (“pig-bel disease”)

◦ Rare in the US

◦ More severe than other form of illnesses and often fatal

◦ Sources:

◦ Raw meats and poultry

◦ Dried or pre-cooked foods

◦ Prepared food in large quantities and kept warm a long time

◦ Vegetable products

◦ Spices and herbs

This Photo by Unknown Author is licensed under CC BY-SA

C. perfringens under the microscope

This Photo by Unknown Author is licensed under CC BY-SA

C. Perfringens on egg yolk agar plate

All Clostridium

species have

peritrichous flagella

with an exception of

C. perfringens

This Photo by Unknown Author is licensed under CC BY-SA

Spores can persist in soil,

sediments, and areas subject to

human or animal fecal pollution

Campylobacter◦ Non-spore forming, gram negative, rod with a curve or S

shaped morphology, and a flagellum at one or both ends of the

bacterium.

◦ Are microaerophilic (grow at lower than atmospheric oxygen

concentration), causing them to be fragile in an ambient

environment.

◦ More than 80% of the infection are caused by C. jejuni.

◦ Common in low-resource countries

◦ Many animals can show no signs of illness as the bacteria is

carried throughout the body and can be transferred to edible

parts when an animal is slaughtered.

◦ Fruits and vegetables can be contaminated through contact

with the soil or water containing feces from other animals.

◦ Sources:

◦ Improper handling or undercooked poultry products

◦ Unpasteurized milk and cheese

◦ Contaminated water

◦ Vegetables

◦ Seafood

This Photo by Unknown Author is licensed under CC BY-SA

This Photo by Unknown Author is licensed under CC BY-NC

Blood Agar Plate

Campylobacter Bacteria

Third leading

bacterial cause

of foodborne

illness in the

U.S.

Pets can carry the

Campylobacter

bacteria and other

germs that can make

you sick.

Staphylococcus aureus

◦ Gram positive, non-motile, small spherical bacteria, appears in

chains or grape like clusters, and can produce heat stable

enterotoxins.

◦ A gastrointestinal illness caused by eating food contaminated

with toxins produced by Staph bacteria

◦ Staph bacteria can be killed in cooking, but the toxins are not

destroyed which will still be able to cause illness.

◦ Food contaminated with the toxin may not smell bad or look

spoiled.

◦ Common contamination by Staph is due to the handling of

food with unwashed hands

◦ About 25% of people and animal have Staph on their skin and

in their nose, which usually does not cause illness.

◦ Source:

◦ Widely distributed in the environment (Ex: air, dust, food, surfaces, animals, humans, etc.)

◦ Meat, meat products, poultry, and egg products

◦ Bakery and dairy products

◦ Contamination when handling food products

This Photo by Unknown Author is licensed under CC BY-SA-NC

This Photo by Unknown Author is licensed under CC BY-SA

Staph bacteria under microscope

Staph colonies on a TSA plate

Staphylococci are

mesophilic and the

optimum temperature

of growth for S.

aureus is 35°C

Strains of S.

aureus are highly

tolerant to salts and

sugars

Escherichia coli (E.coli)◦ Toxin-producing Shiga-toxigenic E.coli (STEC) are Gram

negative, rod-shaped bacteria, with peritrichous flagella.

◦ A subset of STEC which is enterohemorrhagic

Escherichia coli (EHEC) includes O517:H7 which is the

predominant strand that accounts for 75% of EHEC

infections worldwide.

◦ E.coli 0517:H7 is common in North America and was first

identified in 1982.

◦ Less is known about non-O517 STEC subgroups but

cause a less severe illness than O517.

◦ Some people can get less serious infections (range to no

symptoms to diarrhea) but the infection can progress into

life threating form of the illness such as kidney failure.

◦ Sources:

◦ Raw or undercooked ground beef

◦ Unpasteurized milk or fruit juice

◦ Acidic foods

◦ Contaminated water

◦ Vegetables (Ex: sprouts, lettuce, and spinach)

This Photo by Unknown Author is licensed under CC BY-ND

This Photo by Unknown Author is licensed under CC BY

E.coli and M. luteus on TSA plate

E. coli bacterium

Around 5-10% of

those diagnosed with

STEC can develop

hemolytic uremic

syndrome (HUS)

which effects the

kidneys.

STEC live in the gut

of ruminant animals

and the major source

for human illnesses is

cattle. STEC does not

generally make

animals sick but they

are able to pick up

STEC from the

environment and may

spread it.

Listeria monocytogenes

◦ Gram positive, rod-shaped, facultative bacterium, and motile by flagella.

◦ Leading cause of death from foodborne illness. (3rd in the US)

◦ Is salt tolerant, can grow and survive in temperatures below 1°C.

◦ Can cause two forms of disease:

◦ Non-invasive: gastrointestinal illness that resolves

◦ Invasive form: may cause septicemia and meningitis

◦ Pregnant women, people with weak immune systems and the elderly are more susceptible to infection than most other people

◦ Source:

◦ Moist environments, soil, and decaying vegetation

◦ Raw milk, cheese, ice-cream

◦ Raw poultry and meats

◦ Contaminated surfacesThis Photo by Unknown Author is licensed under CC BY-SA

This Photo by CDC website

Listeria monocytogenes bacteria

Colonies on blood agar plate

Pregnant women are 10 times

more likely than other people to

get the infection. It can pass on

to their newborns and may

cause miscarriages, stillbirths,

preterm labor, serious illnesses

and even death.

Foods to avoid according to the CDC:

• Queso fresco and other soft

cheeses

• Raw sprouts

• Melons

• Hot dogs, lunch meat and cold cuts

• Smoked meats

• Unpasteurized milk

Food Fermentation

◦ Microbes play a positive role in food by changing its properties so that it is beneficial.

◦ Majority of fermented foods are produced by:

◦ lactic acid bacteria (Ex: fermented meats, vegetables, and dairy products)

◦ Lactobacillus species

◦ Leuconostoc species

◦ Pediococcus species

◦ Yeast

◦ Saccharomyces cerevisiae (used in bread products)

◦ Kluyveromyces marxianus (dairy products)

◦ The group of organisms can grow under conditions of low pH and reduce water activity.

◦ Lactic acid bacteria have antimicrobial activity since they can inhibit other micro-organisms to improve quality and safety of food products (act as a food preservative).

◦ These foods consumed are termed “probiotics” since they help balance gut bacteria and improve health.

This Photo by Unknown Author is licensed under CC BY-SA

Élie Metchnikoff is a

Russian zoologist, the

founder of the theory

phagocytic immunity,

and studied probiotics,

fermented dairy foods

and health. Élie Metchnikoff circa 1908

Examples of fermented foods

Prevention◦ 4 steps at home for food safety

◦ Clean (wash hands and surfaces often)

◦ Separate (raw food products away from ready to eat foods)

◦ Cook (temperature is important to kill harmful bacteria)

◦ Chill (refrigerate and thaw foods safely)

◦ Always wash your hands to prevent harmful germs getting onto the food

◦ Cook food thoroughly at high enough temperatures to kill germs

◦ Do not eat raw batter or dough and other foods with uncooked eggs or flour

◦ Wash fruits and vegetables before consuming

◦ Bacteria can multiply rapidly if left at room temperature or in the Danger Zone between 40-140F. Do not leave food out for more than 2 hours (or 1 hour if its hotter than 90F outside)

◦ Keep an eye out for recalled food products and immediately dispose of them

◦ More info about food safety and recalls visit: https://www.cdc.gov/foodsafety/index.htmlhttps://www.fda.gov/safety/recalls-market-withdrawals-safety-alerts

ReferencesAdams, M., Moss, M., and McClure, P. (2016). Food Microbiology 4th edition. The Royal Society of Chemistry.

CDC (n.d). Food Safety. https://www.cdc.gov/foodsafety/foodborne-germs.html

Food and Drug Administration (2012). Bad Bug Book, Foodborne, Pathogenic Microorganism and Natural Toxins. 2nd edition.

https://www.fda.gov/media/83271/download

Food and Agriculture Organization of the United Nations (n.d.). Food Loss and Food Waste. http://www.fao.org/food-loss-and-food-waste/en/

Hammond, A. (2019 June 2). Louis Pasteur: "The Father of Microbiology" Who Pioneered Vaccine Science. Foundation for Economic Education.

https://fee.org/articles/louis-pasteur-the-father-of-microbiology-who-pioneered-vaccine-science/

MicroscopeMaster (n.d). Clostridium. https://www.microscopemaster.com/clostridium.html

Osborne, S. (2016 October 24). Antoni van Leeuwenhoek 384th birthday: 'Father of microbiology' celebrated with Google Doodle. Independent.

https://www.independent.co.uk/news/people/antoni-van-leeuwenhoek-google-doodle-384-birthday-father-microbiology-anton-antonie-

a7377201.html

Pelczar, M. and Pelczar, R. (n.d.). Microbiology. Britannica. https://www.britannica.com/science/microbiology