Workbook in Microbiology 7e

Figure 15.2 Penicillin G (shown) and many of its derivatives are inactivated by a beta-lactamase (penicillinase). The enzyme breaks open the beta-lactam ring, which is a common part of the molecular structure of these antimicrobial agents.

Beta-lactam ring confined to certain strains of staphylococci and gram-negative bacilli, it is now found in some strains of bacteria that previously were considered to be universally susceptible to penicillin or its derivatives. These include Haemophilus influenzae, a cause of severe infections in children, before an effective vaccine became available, and Neisseria gonorrhoeae, the agent of gonorrhea.

Bacterial enzymes can also be responsible for resistance to antimicrobial agents other than penicillin. Gentamicin and chloramphenicol, for example, may be inactivated by enzymes specific for these drugs, but there are additional mechanisms by which bacteria may resist the action of certain antimicrobial agents. These include alterations in critical bacterial enzymes or proteins such that they can no longer be directly affected by the drug; or changes in the bacterial cell wall or membrane that make the cell less permeable, preventing entrance of the agent.

Routinely, the clinical microbiology laboratory tests for bacterial susceptibility or resistance by the methods described in Experiments 15.1 and 15.2. Alternatively, however, if you are interested only in the response of a given organism to a particular antimicrobial agent (e.g., Neisseria gonorrhoeae to penicillin), you can test the organism for its ability to produce a sufficient amount of an enzyme that specifically inactivates that drug. If the organism can be shown to possess the enzyme, it is considered to be resistant to the antimicrobial agent in question. One such test is illustrated in the following experiment, using a penicillin-susceptible organism and one that is resistant to penicillin because it produces penicillinase. The test uses a filter paper disk containing the chromogenic (color-producing) cephalosporin, nitrocefin. Like penicillin, the cephalosporins are degraded by beta-lactamases. When the test disk is inoculated with a penicillinase-producing organism, the yellow nitrocefin is broken down to a red end product.


To detect penicillinase production by a test bacterial strain


Filter paper disks impregnated with nitrocefin for performing the beta-lactamase test

Sterile water or saline

Clean glass slides


Plate culture of a penicillin-resistant Staphylococcus aureus

Plate culture of a penicillin-susceptible Bacillus subtilis

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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