A Rapid Procedure for Screening Unusual Penicillin-resistant Clinical Strains of Staphylococci

Abstract

THE increased number of resistant Staphylococci occurring in many parts of the world has led to the recent introduction of novel penicillins^1–3. These penicillins are clinically effective against strains producing penicillinase (β-lactamase)^4,5. However, it has been shown that another means of resistance to penicillin, namely acylase activity, exists in some bacteria⁶. Knox and Smith⁷ introduced a rapid screening test, differentiating penicillinase producers from non-producers in a mixed population, based on the detection of acid formed from penicillin by penicillinase. This was rather non-specific, since acid could also be formed if any acylase were operative⁸. In consideration of this possibility, Foley and Perret⁹ reported a further test which involved absorption of iodine by penicilloic acid formed in the presence of β-lactamase. They suggested that the production of acid could be effected by metabolic products, whereas iodine absorption would be a reaction specific for the detection of penicilloic acid. It may be possible, nevertheless, that some non-specific iodine absorption occurs when a very low concentration of iodine is used¹⁰. Therefore, it seemed that some modifications in these methods of testing resistant strains were needed. Furthermore, such modifications should be able to differentiate between β-lactamase and any other mechanism(s) of resistance. The presence of another mechanism might render ineffective the action of the novel penicillins. Thus, resistant mutants of clinical importance may be selected out, as has occurred with penicillin G. Methicillin resistance in Staphylococci has already been reported to be on the increase¹¹.