Quantitative determination of beta-lactamase production in Staphylococcus aureus strains compared to qualitative testing by a microbiological clover leaf test, a chromogenic cephalosporin test and a iodometric test.

One hundred and twelve beta-lactamase producing strains of Staphylococcus aureus with known quantitative production were investigated together with 45 non-beta-lactamase producing derivatives by three qualitative beta-lactamase tests: the microbiological clover leaf test, the iodometric tube test and a chromogenic cephalosporin disc test. For the beta-lactamase producing strains strong positive reactions in the qualitative tests were not correlated with high beta-lactamase production, large degree of extracellularity or high induction ratio in any of the tests. Low beta-lactamase production was not correlated to weak reactions. Best results in identifying beta-lactamase production were obtained using the iodometric test in which 109 of 112 beta-lactamase producing strains were positive and no false positive reactions were observed. Some strains had only weak positive reactions but they were easily read. The clover leaf test and the chromogenic cephalosporin test detected 105 and 107 of 112 strains, respectively. The chromogenic cephalosporin disc test had more weakly reacting strains, which were difficult to read, especially among strains of phage type 95. Three non-beta-lactamase producing strains of phage type 95 were positive in the clover leaf test. It is concluded that the iodometric tube test is the most useful one for qualitative determination of beta-lactamase production in S. aureus. It is recommended as control when normal susceptibility testing for penicillin has shown susceptibility or doubtful susceptibility. Furthermore, it is useful when no susceptibility testing has been carried out and a rapid result is needed.