Food Matters

Spud's Down to Earth Advice on...

MICROBIOLOGY

• pathogenic bacteria - harmful to humans. NB. These rarely alter the taste, smell or appearance of food
• spoilage bacteria - cause unacceptable deterioration of food by breaking down protein. They do not usually cause food poisoning.
• commensal bacteria - generally harmless, some can be beneficial

• by shape and features
• by anatomy i.e. the microscopic structure of the bacterial cell itself
• by physiology i.e. particular features of the way it lives (its metabolism) and the conditions it favours

• an 'envelope' (comprising structures outside the cell membrane, such as the cell wall and any external structures)
• the cytoplasmic (cell) membrane
• the cytoplasm

• it provides strength and shape
• it protects the cell from damage from the adverse movement of water and from toxic substances. This therefore provides an ideal target site for anti-bacterial agents.
• it may have components that contribute to the harm they cause humans e.g. in Gram-ve bacteria there is an outer membrane (made from lipopolysaccharides) outside the cell wall that can cause harm as 'endotoxins' (see below).

• Capsule: a gel-like outer layer that may protect the cell against drying out or attack.
• Slime layer: similar to the capsule but less tightly bound to the cell and therefore easier to remove. NB. When bacteria with a capsule or slime layer contaminate food, the surface of the food becomes slimy or sticky. Such bacteria can also form 'biofilms' on food contact surfaces that can be difficult to remove.
• Flagellum (plural = flagella): these are long strands attached to the cell that act like a sort of propeller to move the cell around, in response to chemical changes in the cell's environment. Bacteria may have more than one flagella (monotrichous = 1, amphitrichous = 1 at each end of the cell, lophotrichous = 2 or more at each end, peritrichous = all over [e.g. Salmonella and E. coli]
• Fimbriae: present in many Gram-ve bacteria; they are very small, short, fine, hair-like appendages. They are not involved in movement of the cell but play a part in attaching the cell to surfaces, including that of the human intestine, and as a result can be a determining factor in causing harm to humans (e.g. E. coli).

• Bacteria responsible for causing food poisoning need certain conditions in order to grow and, in some cases, produce toxins.
• Provided conditions are right, bacterial cells reproduce (multiply) by a process known as binary fission. In this process, a single bacterial cell divides into two new but identical cells. These in turn grow to maturity and then further divide into new bacterial cells.
• The time between each cell division (the generation time) varies between bacteria and is dependent on the conditions, e.g. Clostridium perfringens = 8 minutes, E. coli = 20 minutes. Just imagine - in this way a single cell can result in more than a million bacteria in under 3 hours. This amazing figure should also be seen alongside the fact that the infective dose (the number of bacteria required to produce illness) varies, from perhaps as few as 100 cells for E. coli O157 to perhaps one million cells for Salmonella.
• Although generation times may vary, all bacterial populations follow the same general pattern of growth phases over a period of time:

• Lag phase: initial period when there is little or no growth, as the bacteria 'get used' to their environment
• Log phase: where there is rapid reproduction and an sharp rise in the number of bacteria present
• Stationary phase: where there is a balance in the numbers of bacteria dying and those growing.
• Decline phase: where more cells are dying than being produced

• Bacteria like food sources that unfortunately are also consumed by humans. They like foods that are rich in proteins and carbohydrates, from which they obtain their essential nutrients.
• 'High risk' foods are those that offer a particularly attractive environment to bacteria that can then, given favourable conditions, multiply rapidly e.g. meat and poultry (and meat and poultry products), eggs, milk and dairy products, seafood.
• Growth of bacteria in certain foods can be repressed by the presence of high concentrations of salt or sugar, which serve to bind to and reduce the amount of water available to the bacteria for growth.
• A bacterium's ability to survive and/or thrive in acidic environments varies between species and is also dependent on other environmental conditions, e.g. temperature. Although bacteria generally prefer neutral pH (7) most can be found in the range 4.5 - 9 (slightly acidic to slightly alkaline). As most foods have a pH between 7 (neutral) and 4 (slightly acidic) they can therefore support bacterial multiplication. Equally, bacteria will not grow in foods with a pH below 4 (e.g. carbonated drinks and most fruits). Lower pH (higher acid) foods inhibit bacterial growth by:

• disrupting of the working of the cell membrane
• inhibiting the activity of bacterial enzymes, or
• ionising (giving an electrical charge to) the nutrient molecules thereby reducing their availability to the organism.

• Bacteria can multiply at temperatures between 5^oC and 63^oC, a temperature range commonly referred to as the danger zone. Growth tends to be faster between 20^oC and 50^oC, and for many food poisoning bacteria is most rapid at about 35-37^oC.
• Bacterial growth slows down the colder it gets, and stops altogether in very cold conditions. Therefore most food poisoning bacteria will not grow in a refrigerator (1^oC to 4^oC) and none at all will grow in

• As a survival mechanism when faced with extreme adverse conditions e.g. dehydration, disinfection and heat, some Gram+ve bacteria, including the two food pathogens Bacillus species and Clostridium species, can develop a specially resistant, dormant structure called an endospore, commonly referred to as a spore. Spores can remain dormant for long periods and are extremely resistant to environmental stresses - indeed, it may take temperatures of above 100^oC to kill them.
• Foodborne illness can arise if food that contains spores is improperly stored or handled so that conditions become favourable for bacterial multiplication long enough for the spores to germinate, releasing active cells. These bacteria can then reproduce rapidly. Foodborne illness can subsequently occur due to the food containing dangerous levels of bacteria or bacterial toxins (see below).

• heat stable
• toxic only at high doses
• usually capable of producing general systemic effects (fever, shock, weakness, diarrhoea, inflammation etc.)