A Microbiology Technologist Is Performing A $\beta$ -Lactamase Test on a Staphylococcus Isolated from a Hospitalized Patient

A microbiology technologist is performing a $\beta$ -lactamase test on a staphylococcus isolated from a hospitalized patient. The test was negative, so the technologist reported out in vitro antimicrobial susceptibility test results for $\beta$ -lactam antibiotics. After 2 days, the patient was not improving and the physician changed the patient's antibiotic to an aminoglycoside. The patient showed improvement immediately. Why was the $\beta$ -lactamase test on the staphylococci negative, when the organism produced $\beta$ -lactamase?

A) The test was a false negative-the organism really did produce $\beta$ -lactamase, but it did not react with the test.
B) Staphylococci must be exposed to the $\beta$ -lactam agent before it will produce a positive test result.
C) The test was a false positive-the organism reacted with the test, but it did not produce a $\beta$ -lactamase.
D) The staphylococci on the plate were more than 24 hours old and false-negative test results can occur when an "old" organism is used.

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