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Basic aspects

Diversity of microbial Life

There is considerable diversity in water and food borne microorganisms which are capable of causing human disease. Although bacteria have been the main focus of attention it should be recognised that this is primarily due to the relative ease with which these organisms can be cultured compared with viruses. In fact the small round structured viruses which caused such considerable numbers of viral gastroenteritis cannot yet be cultured in the laboratory. A review of waterborne diseases from 1991-2000 in the USA has been released and has copious amounts of information (URL waterborne diseases USA)).

Bacterial cell structure

It is important to understand at least the basic structure of the microbial cell wall. The difference in structure between Gram negative and Gram positive organisms explains the staining reactions and this is a basic technique used in microbiology. The detailed structure of the outer cell membrane of Gram negative organisms (Fig 2.2 and 2.3 , p56 and 57) demonstrates the usefulness of serology in identifying isolates, especially in epidemiological studies. If you do not understand the 'O157:H7' term used for certain pathogenic strains of E. coli then study again the cell wall structure.

Microbial growth curves

Bacterial multiplication is often represented by the classic growth curve (Fig 2.9, p65). The curve is divided into several phases; lag, exponential (or logarithmic), stationary and death. To review how bacteria grow you can visit the Cells Alive Web site.

It is important to understand the factors affecting these phases as they determine whether an organism is capable of growing in food. Ultimately it may be possible to accurately predict the extent of microbial growth and this topic is covered in more detail later. An example of predictive microbiology is Microfit (UK) for which you enter in your viable counts at known time intervals and the programs predicts the starting inoculum size, lag period, doubling time and maximum number of cells.

Rates of microbial death, D and Z values

There are a number of terms used to describe microbial death:

  • D value
  • Z value
  • P value
  • F value

The D value is the 'decimal reduction' time. This is the time required at any given temperature to reduce the viable count by 1 log order (in other words 90%). Although 'maths' might not be your forte, determining D and Z values is not too difficult. There are some examples given in Fig 2.10 & 2.12 (p67&69) as well as tabulated ones (Table 2.3, p68).

Food additives and contact materials

I had not majored on food additives nor contact materials (active packaging) in the books, but here are some useful web sites if you are interested.

Intrinsic and extrinsic factors affecting microbial growth

Factors affecting microbial growth can be divided into two categories; intrinsic and extrinsic (Table 2.7, p72). These can be reviewed at the FDA (USA) Bad Bug Book. Water activity, pH and temperature are the most important factors However water activity and pH are not always that easy to measure. The reason being that water activity is not the same as moisture, it is the amount of 'unbound' water that is available to the microorganisms. Additionally food being so heterogeneous means that different regions of a food will have different water activity values and should not be simply 'averaged'. Similarly the pH of a food (web site) can vary from zone to zone.

A general site covering can be found at this Web site.

Predictive Microbiology

Since bacterial growth follows a mathematical pattern it is possible to develop computer programs which will predict the rates of bacterial growth and death under defined environmental conditions (ARTICLE). A number of computer software programs are available, such as:

  1. Food Micromodel (UK)
  2. Pathogen Modeling Program (PMP) (USA)
  3. Combase; Common database for foodborne bacteria
The Pathogen Modeling Program can run from the USDA web site. However used an earlier downloaded version for the book, the main difference only the updated use of the WindowsTM display environment and not the data generated.

Society for Applied Microbiology (SfAM) Food Safety Summit