Susceptibility Of Pharmaceutical Preparations To Microbiological Problems

Two problem, broadly speaking, are created by the presence of microorganisms in pharmaceutical preparations. First, they may harm the patient by causing infection. Second, they may alter the composition of the preparation to the extent that it may not function in the way it was intended, or that the patient may reject it as "spoiled."

Generally (but not invariably), high numbers of microorganisms are necessary for either problem to arise in, or from, a pharmaceutical preparation. Two processes must occur to allow microbial contaminants to reach problematic numbers:

• There must be an initial contamination of the preparation by microorganisms

• The microbial contaminants must proliferate in the preparation, and must metabolise, grow and multiply

The probability of any initial contamination arising is a reflection of how well good manufacturing practices (GMPs) have been applied. However, the likelihood of proliferation is largely a function of the composition of the preparation itself. There are three alternative fates for microorganisms contaminating pharmaceutical preparations (Figure 4.1):

• They may survive without proliferating

• They may metabolize, grow and multiply

(The fourth remote possibility is that microorganisms might "escape.")

Although microbiology is not a very precise science, it can quite confidently predict the most likely fate of contaminants in various types of pharmaceutical preparation. Microorganisms require water to metabolize, grow and multiply.

They survive

Figure 4.1. Alternative fates of microbiological contaminants.

Figure 4.1. Alternative fates of microbiological contaminants.

Strictly speaking they require "free" or "unbound" water as measurable through water activity.1'4,8 There is only a very low probability of microbial proliferation in nonaqueous pharmaceutical preparations. Their most likely fate is death through desiccation — at worst, desiccation-resistant types (e.g., Bacillus, Micrococcus) survive without multiplying. The greatest opportunity areas for microbiological proliferation are in aqueous-based pharmaceutical preparations.

Of all aqueous-based preparations, inhalations, oral solutions and suspensions, topical lotions and creams are most susceptible to microbiological problems (Table 4.1). Syrups and topical gels are of medium susceptibility. Ointments are of quite low susceptibility to microbiological problems. Correspondingly these differences in susceptibility to microbiological problems define the attention that should be given to addressing microbiological concerns in formulation, in the choice of starting materials and in the control of manufacture.

Table 4.1. Susceptibility (Based on Water Content) of Pharmaceutical Preparations to Microbiological Problems

Low Susceptibility Medium Susceptibility High Susceptibility

Ointments Oral syrups Aqueous inhalations

Topical gels

Oral solutions Oral suspensions

Topical lotions Topical creams

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