Physicochemical Factors that affect Growth and Survival of Bacteria

PHYSICOCHEMICAL FACTORS THAT AFFECT GROWTH AND SURVIVAL OF BACTERIA

Temperature

Earlier in this chapter various classes of bacteria (thermophile, mesophile, etc.) were described according to the range of temperatures under which they could grow. The majority of bacteria that have medical or pharmaceutical significance are mesophiles and have optimal growth temperatures between ambient and body temperature (37 °C). Individual species of bacteria also have a range of temperatures under which they can actively grow and multiply (permissive temperatures). For every organism there is a minimum temperature below which no growth occurs, an optimum temperature at which growth is most rapid and a maximum temperature above which growth is not possible (Figure 3.9). As temperatures rise, chemical and enzymic reactions within the cell proceed more rapidly, and growth becomes faster until an optimal rate is achieved. Beyond this temperature certain proteins may become irreversibly damaged through thermal lysis, resulting in a rapid loss of cell viability.

The optimum temperature for growth is much nearer the maximum value than the minimum, and the range of the permissive temperatures can be quite narrow (3–4 °C) for obligate pathogens yet broad (10–20 °C) for environmental isolates, reflecting the range of temperatures that they are likely to encounter in their specialized niches. If the temperature exceeds the permissive range then provided that lethal temperatures are not achieved ( c.60 °C for most Gram-negative mesophiles) the organisms will survive but not grow. Temperatures of 105 °C and above are rapidly lethal and can be deployed to sterilize materials and products. Generally bacteria are able to survive temperatures beneath the permissive range provided that they are gradually acclimatized to them.

pH

As for temperature, each individual microorganism has an optimal pH for growth and a range about that optimum where growth can occur albeit at a slower pace. Unlike the response to temperature, pH effects on growth are bell-shaped (Figure 3.10), and extremes of pH can be lethal. Generally those microorganisms that have medical or pharmaceutical significance have pH growth optima of between 7.4 and 7.6 but may grow sub-optimally at pH values of 5–8.5. Thus growth of lactobacilli within the vaginal vault reduces the pH to approximately 5.5 and prevents the growth of many opportunist pathogens. Accordingly, the pH of a pharmaceutical preparation may dictate the range of microorganisms that could potentially cause its spoilage.

Water activity/solutes

Water is essential for the growth of all known forms of life. Gram-negative bacteria are particularly adapted to an existence in, and are able to extract trace nutrients from, the most dilute environments. This adaptation has its limitations because the Gram-negative cell envelope cannot withstand the high internal osmotic pressures associated with rapid rehydration after desiccation and the organisms are unable to grow in the presence of high concentrations of solute. The availability of water is reflected in the water activity of a material or liquid. Water activity ( A_w) is defined as the vapour pressure of water in the space above the material relative to the vapour pressure above pure water at the same temperature and pressure. Pure water by definition has an A_w of 1.00. Pharmaceutical creams might have A_w values of 0.8–0.98, whereas strawberry jam might have an A_w of c.0.7. Generally Gram negative bacteria cannot grow if the A_w is below 0.97, whereas Gram-positive bacteria can grow in materials with A_w of 0.8–0.98 and can survive rehydration after periods of desiccation, hence their dominance in the soil. Yeasts and moulds can grow at low A_w values, hence their appearance on moist bathroom walls and on the surface of jam. The water activity of a pharmaceutical product can markedly affect its vulnerability to spoilage contaminants.

Availability of oxygen

For many aerobic micro-organisms oxygen acts as the terminal electron acceptor in respiration and is essential for growth. Alternative terminal electron acceptors are organic molecules whose reduction leads to the generation of organic acids such as lactic acid. They can sometimes be utilized under conditions of low oxygen or where carbon substrate is in excess (fermentation), and highly specialized groups of microorganisms can utilize inorganic materials such as iron as electron acceptors (e.g. iron–sulphur bacteria). Different groups of organisms therefore vary in their dependence on oxygen. Paradoxically, there are many bacteria for which oxygen is highly toxic (obligate anaerobes), so the presence or absence of oxygen within a nutrient environment can profoundly affect both the rate and nature of the microbial growth obtained. Strongly oxygen-dependent bacteria will tend to grow as a thin pellicle on the surfaces of liquid media where oxygen is most available. Special media and anaerobic chambers are required to grow obligate anaerobes within the laboratory, yet such organisms persist and actively grow within the general environment. This is because the close proximity of strongly aerobic cells and anaerobes will create an anoxic microenvironment in which the anaerobe can flourish. This is particularly the case for the mouth and gastrointestinal tract where obligate anaerobes such as Bacteroides and Fusobacter can be found in association with strongly aerobic streptococci.

The inability of oxygen to diffuse adequately into a liquid culture is often the factor that causes an onset of stationary phase, so culture density is limited by oxygen demand. The cell density at stationary phase can often be increased, therefore, by shaking the flask or providing baffles. Diffusion of oxygen may also be a factor limiting the size of bacterial colonies formed on an agar surface.