Abstract
The penicillin-starch paper strip method was compared with the acidometric and iodometric methods for assaying beta-lactamase production, using fresh isolates of clinically important bacteria. Results obtained by the three methods were compared for rapidity, accuracy, and stability of reagents. Of the 210 isolates tested by the paper strip method, 301 isolates tested by the acidometric method, and 117 isolates tested by the iodometric method, all were in perfect agreement with the disk diffusion susceptibility test except one strain each of Haemophilus influenzae, Staphylococcus aureus, and Staphylococcus epidermidis. The H. influenzae isolate was penicillin resistant and failed to give a positive test for beta-lactamase in all three tests. The staphylococci (intermediate and resistant in susceptibility, respectively) failed to give a positive test for beta-lactamase with the iodometric method. The results of the paper strip method, in which 3,241 strains representing nine species of bacteria were used, correlated completely with disk susceptibility tests except for 2 and 69 strains, respectively, of penicillin-resistant, beta-lactamase-negative H. influenzae and H. parainfluenzae. The results of this study indicate that the paper strip method is accurate, simple to perform, extremely economical, and uses materials that are stable when stored frozen. It is eminently suitable for routine laboratory use.
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