The clover-leaf test and inactivation of beta-lactam antibiotics by gram-negative rods

The suitability of a very sensitive modification of the clover-leaf test as a test for inactivation of beta-lactam antibiotics by gram-negative rods was examined. 143 Enterobacteriaceae and 9 strains of Pseudomonas aeruginosa were examined by the clover-leaf test and an antibiotic susceptibility test towards 12 beta-lactam antibiotics. The beta-lactamase activity of the strains was also examined by a sensitive modification of the chromogenic cephalosporin test. The strains of P. aeruginosa could not be clover-leaf tested as they inhibited the indicator strains. All the Enterobacteriaceae which had beta-lactamase activity according to the chromogenic test inactivated at least 1 of the antibiotics in the clover-leaf test. None of the Enterobacteriaceae without beta-lactamase activity inactivated any of the antibiotics. On the whole the results of the clover-leaf test and the results of the susceptibility test correlated well. But many Enterobacteriaceae were susceptible to an antibiotic in the susceptibility test even though they inactivated it according to the clover-leaf test and some Enterobacteriaceae were resistant to an antibiotic but did not inactivate it. Among the Enterobacteriaceae which had beta-lactamase activity according to the chromogenic cephalosporin test and which were intermediately susceptible to an antibiotic in the susceptibility test, many did not inactivate this antibiotic according to the clover-leaf test. If an infection with Enterobacteriaceae has to be treated by a beta-lactam antibiotic to which the strain is only intermediately susceptible, it might be advantageous to use an antibiotic which is not inactivated by the strain according to the clover-leaf test, but this lacks clinical confirmation.