Viability is defined as the capability of microorganisms to grow, divide and increase sufficiently to form visible colonies on solid nutrient media, or turbidity or other visible change in fluid nutrient media. Within the need for sterility in parenteral products, the presence of one viable microorganism in a sealed product unit is considered sufficient to potentially cause infection.

However, this is only partially true. The numbers of viable microorganisms in lethally contaminated infusion fluids during the 1970s were, in all cases, in concentrations exceeding 107ml; several hundred millilitres were possibly infused into each patient.

The presence of foreign matter in the body or at the site of injection is known to influence the threshold number of microorganisms required to cause a clinically recognizable infection. Elek and Conen (1957) established that when Staphylococcus pyogenes alone was injected into human volunteers, 106 microorganisms were required to produce a pus-forming infection, but only 102 were required when foreign matter (braided silk suture) was included with the inoculum.

The apparent requirement for a threshold number of microorganisms, which must be exceeded to overwhelm patient defence mechanisms and cause infections, has been confirmed experimentally for Staphylococcus aureus and Gram-negative bacteria.

Other microorganisms may survive in small numbers in the human body; for instance, Streptococcus viridans may generate a protective slime and adhere to diseased natural tissues. Subsequently, perhaps during periods of immunodepression, it may proliferate and establish infections (Dougherty, 1988).

The pharmaceutical industry regards even one viable contaminant in a sealed product unit as a compromise of sterility, regardless of the size of the product unit, whether it be a 0.5-ml subcutaneous injection or a llitre intravenous infusion.

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