• Cocci:
  • spherical shape
  • 0.4 – 1.5 µ m

Examples: staphylococci – form grape-like clusters; streptococci – form bead-like chains.

• Rods:
  • 0.25 – 1.0 µ m width by 0.5 – 6.0 µ m long

Examples: bacilli – straight rod; spirilla – spiral rod

There exists a bacterial system of taxonomy, or classification system, that is internationally recognized with family, genera and species divisions based on genetics.

Some bacteria have the ability to form resting cells known as endospores. The spore forms in times of environmental stress, such as lack of nutrients and moisture needed for growth, and thus is a survival strategy. Spores have no metabolism and can withstand adverse conditions such as heat, disinfectants, and ultraviolet light. When the environment becomes favourable, the spore germinates and giving rise to a single vegetative bacterial cell. Some examples of spore-formers important to the food industry are members of Bacillus and Clostridium generas.

Bacteria reproduce asexually by fission or simple division of the cell and its contents. The doubling time, or generation time, can be as short as 20-20 min. Since each cell grows and divides at the same rate as the parent cell, this could under favourable conditions translate to an increase from one to 10 million cells in 11 hours! However, bacterial growth in reality is limited by lack of nutrients, accumulation of toxins and metabolic wastes, unfavourable temperatures and desiccation. The maximum number of bacteria is approximately 1 X 10e9 CFU/g or ml.

Note: Bacterial populations are expressed as colony forming units (CFU) per gram or millilitre.

Bacterial growth generally proceeds through a series of phases:

• Lag phase: time for microorganisms to become accustomed to their new environment. There is little or no growth during this phase.
• Log phase: bacteria logarithmic, or exponential, growth begins; the rate of multiplication is the most rapid and constant.
• Stationary phase: the rate of multiplication slows down due to lack of nutrients and build-up of toxins. At the same time, bacteria are constantly dying so the numbers actually remain constant.
• Death phase: cell numbers decrease as growth stops and existing cells die off.

The shape of the curve (shown on the right) varies with temperature, nutrient supply, and other growth factors. This exponential death curve is also used in modeling the heating destruction of microorganisms.

Yeasts

Yeasts are members of a higher group of microorganisms called fungi . They are single-cell organisms of spherical, elliptical or cylindrical shape. Their size varies greatly but are generally larger than bacterial cells. Yeasts may be divided into two groups according to their method of reproduction:

1. budding: called Fungi Imperfecti or false yeasts
2. budding and spore formation: called Ascomycetes or true yeasts

Unlike bacterial spores, yeast form spores as a method of reproduction.

Moulds

Moulds are filamentous, multi-celled fungi with an average size larger than both bacteria and yeasts (10 X 40 µ m). Each filament is referred to as a hypha. The mass of hyphae that can quickly spread over a food substrate is called the mycelium. Moulds may reproduce either asexually or sexually, sometimes both within the same species.

Asexual Reproduction:

• fragmentation – hyphae separate into individual cells called arthropsores
• spore production – formed in the tip of a fruiting hyphae, called conidia, or in swollen structures called sporangium

Sexual Reproduction: sexual spores are produced by nuclear fission in times of unfavourable conditions to ensure survival.

Microbial Growth

 

There are a number of factors that affect the survival and growth of microorganisms in food. The parameters that are inherent to the food, or intrinsic factors, include the following:

• nutrient content
• moisture content
• pH
• available oxygen
• biological structures
• antimicrobial constituents

Nutrient Requirements

While the nutrient requirements are quite organism specific, the microorganisms of importance in foods require the following:

• water
• energy source
• carbon/nitrogen source
• vitamins
• minerals

Milk and dairy products are generally very rich in nutrients which provides an ideal growth environment for many microorganisms.

Moisture Content

All microorganisms require water but the amount necessary for growth varies between species. The amount of water that is available in food is expressed in terms of water activity (aw), where the aw of pure water is 1.0. Each microorganism has a maximum, optimum, and minimum aw for growth and survival. Generally bacteria dominate in foods with high aw (minimum approximately 0.90 aw) while yeasts and moulds, which require less moisture, dominate in low aw foods ( minimum 0.70 aw). The water activity of fluid milk is approximately 0.98 aw.

pH

Most microorganisms have approximately a neutral pH optimum (pH 6-7.5). Yeasts are able to grow in a more acid environment compared to bacteria. Moulds can grow over a wide pH range but prefer only slightly acid conditions. Milk has a pH of 6.6 which is ideal for the growth of many microoorganisms.

Available Oxygen

Microorganisms can be classified according to their oxygen requirements necessary for growth and survival:

• Obligate Aerobes: oxygen required
• Facultative: grow in the presence or absence of oxygen
• Microaerophilic: grow best at very low levels of oxygen
• Aerotolerant Anaerobes: oxygen not required for growth but not harmful if present
• Obligate Anaerobes: grow only in complete absence of oxygen; if present it can be lethal

Biological Structures

Physical barriers such as skin, rinds, feathers, etc. have provided protection to plants and animals against the invasion of microorganisms. Milk, however, is a fluid product with no barriers to the spreading of microorganisms throughout the product.

Antimicrobial Constituents

As part of the natural protection against microorganisms, many foods have antimicrobial factors. Milk has several nonimmunological proteins which inhibit the growth and metabolism of many microorganisms including the following most common:

1. lactoperoxidase
2. lactoferrin
3. lysozyme
4. xanthine

More information on these antimicrobials can be found in the dairy microbiology textbook by Marth and Steele. See also the discussion on lactoperoxidase in this series here.

Where the intrinsic factors are related to the food properties, the extrinsic factors are related to the storage environment. These would include temperature, relative humidity, and gases that surround the food.

Temperature

As a group, microorganisms are capable of growth over an extremely wide temperature range. However, in any particular environment, the types and numbers of microorganisms will depend greatly on the temperature. According to temperature, microorganisms can be placed into one of three broad groups:

• Psychrotrophs: optimum growth temperatures 20 to 30° capable of growth at temperatures less than 7° C. Psychrotrophic organisms are specifically important in the spoilage of refrigerated dairy products.
• Mesophiles: optimum growth temperatures 30 to 40° C; do not grow at refrigeration temperatures
• Thermophiles: optimum growth between 55 and 65° C

It is important to note that for each group, the growth rate increases as the temperature increases only up to an optimum, after which it rapidly declines.