Food Microbiology

Most of the micro-organisms associated with food microbiology fall into one of the following groups -
Viruses
Bacteria
Parasites
Fungi

Viruses – much smaller than bacteria. Cannot be seen by normal light microscopy. We would need to use an Electron microscope.

A virus is made up of a core of nucleic acid surrounded by a protein coat.

Cannot grow in food but lay dormant in food and water. When ingested they can become active.

An example of a virus is Hepatitis A (associated with shellfish). Shellfish are filter feeders and tend to live near sewage outfall. This concentrates the viruses which may be in the shellfish. This may present a problem for example for people eating raw oysters.

Norovirus
Lace-like appearance of individual virus particles. The virus particles of Norwalk and other
SRSVs (Small Round Structured Viruses)

Rotavirus
Wheel-like appearance. The observance of such particles gave the virus its name
('rota' being the Latin word meaning wheel).

Rotavirus more commonly causes vomiting and illness in children. It is thought that most people build a resistance to it.



Bacteria

Bacteria are minute, they cannot be seen without a microscope.

Only a small proportion are pathogenic most are Commensal flora. There are thousands of different types. Many perform useful functions.

One of the most distinguishing features of bacteria is their reaction to Grams stain. In 1884, Hans Christian Gram, a Danish doctor working in Berlin, accidentally stumbled on a method which still forms the basis for the identification of bacteria. While examining lung tissue from patients who had died of pneumonia, he discovered that certain stains were preferentially taken up and retained by bacterial cells. Over the course of the next few years, Gram developed a staining procedure which divided almost all bacteria into two large groups - the Gram stain

Gram stains fall into two groups – positive and negative
This is just a way of categorising bacteria.

Round shapes are cocci
Rod shaped are bacilli

Some bacteria are motile in fluids and can swim or dart about by means of flagella
Polar flagella at the end of the organism – these bacteria dart about ( a bit like tadpoles)
Bipolar – flagella at both ends, so they tumble about.
All round flagella – peritrichous – so they move about like a corkscrew or rifle bullet.

All bacteria are classified to genus and species eg Staphyloccocus aureus – Staphyloccocus is the genus, aureus is the species.


Other important cell components (not in all bacteria) include capsules and spores.

Capsules form a protective covering. Spores are formed when conditions are not favourable (eg lack of moisture). Spores are formed within the bacterial cells (endospores). When cell disintegrates, the spores remain intact. Can withstand heat, light, chemicals, lack of moisture etc. germinate when conditions are favourable.

Bacteria divide by simple division (into 2). Divide by binary fission and each generation is doubled in number (2,4,8,16,32 etc). Under suitable conditions may occur every 15 to 20 mins (therefore > million in 4 hours)?


Growth occurs in 3 stages

Lag – can grow but slowly – a period of adaptation to the environment
Logarithmic/exponential – constant specific growth rate (binary fission). Dependent on conditions, temp, time etc

Stationary – one or more of the nutrients becomes exhausted or toxic metabolic products accumulate and inhibit growth. Growth ceases. Number of viable bacteria remains constant

Decline/death – death rate exceeds the rate of multiplication. Majority of cells die. Some may form spores.


Temperature - important factor as it influences the rates of all chemical reactions linked to the process of growth. The temperature at which a culture medium is incubated determines the rate of growth of any bacteria associated with it. For any organism the temperature at which growth is most rapid is known as the optimum temperature.
The minimum temperature is the lowest temperature at which growth occurs (usually substantially below the optimum temperature)
The maximum temperature is the highest temperature at which the organism will grow. Usually only a few degrees above the optimum.

Bacteria can be distinguished on the basis of their temperature relationships
Psychrophiles – grow at very low temperatures. Optimum 5-20C and ability to grow at 0C (and as low as –22C). Rare group but include some spoilage bacteria (eg some species of pseudomonas)

Psychrotrophs - Yersinia enterocolitica is a psychrotrophic organism (can grow at fridge temps)
Listeria another psychrotrophic organism

Theromphiles prefer 45 to 70C

Hyperthermophiles – above 75C

Mesophiles – most food borne pathogens are mesophiles and grow best at 30 to 45C


Moisture – at least 80% bacterial cell consists of water. Essential for growth and survival
Bacteria vary on the dependency on water.

Atmosphere – Bacteria differ in their need for oxygen for growth, or in its exclusion. Bacteria that are dependent on free oxygen present in air are called obligate or strict aerobes. (eg Pseudomonas spp)
Bacteria that will only grow in the absence of oxygen are called obligate anaerobes. Even traces of Oxygen are toxic to them (eg Clostridia spp)
Majority of bacteria fall between these two extremes and are called facultative anaerobes (eg E coli, S aureus). Able to grow anaerobically but prefer aerobic conditions.

AEROBIC – grows in air
MICROAEROPHILIC – grows in reduced oxygen
FACULTATIVE – grows with or without oxygen
ANAEROBES – cannot grow if oxygen is present

PH – or hydrogen ion concentration. Significant effect on the growth of bacteria. All bacteria have an optimum pH. Most bacteria favour a neutral pH (6.8 to 7.5). Some prefer alkaline conditions (eg vibrio like 8.5 to 9.0, yeasts and moulds like alkaline conditions)

Time – most bacteria can form cfu within 18hours on agar under optimum conditions. But some need 48hrs before producing typical morphology. (in fact Legionella can take up to 10 days to grow – useful as contaminants can grow in 24 hours). Moulds can take up to 5 days

Micro-organisms are everywhere

Not all of them are pathogenic. Some of them are very useful. bacterial growth and survival depends on several factors. These include nutrients, pH, time, temperature.
Bacteria are able to survive harsh conditions (eg drying – think of spices). This can make them very difficult to control.


Biofilms: Biofilms are another way in which bacteria can protect themselves under harsh conditions. Biofilms can be very difficult to remove I have given you some example of biofilms on the slide

Biofilms are a problem on rough surfaces (think about chopping boards, serving areas)
Biofilms are of an organic nature (ie they are made up of bacteria) which may or may not be coated in substances such as food. They may lie dormant waiting for a trigger (eg moisture, warmth, food) to make them grow.


The micro-organism in a food does not have to grow to cause problems.
Some micro-organisms use the food as a vehicle of transmission

Some example of these are Viruses (the chef may have a viral infection – he is suffering from vomiting – he manages to get some of the virus onto the food he is preparing – his customers also become infected with the virus)
Another example are parsites such as Giardia which may be passed on via food from poor hygiene in the kitchen (eg an infected food handler does not wash his hands properly after going to the toilet)
Campylobacter does not grow in food but survives well. In the laboratory were I work we are finding that about 90% raw chicken is infected with campylobacter.


If the conditions are favourable (temperature, humidity, etc), and the organism is a pathogen
We may get food poisoning We can get food poisoning by an infective dose (ie we actually eat the bacteria in sufficient numbers to cause illness – this can be a very low number in some organisms such as E.coli O157) Or, we can become ill by ingesting the toxin which has been produced by the bacteria in the food (eg Staphylococcus aureus toxin which can cause severe vomiting within hours)