Penicillin resistance and B-lactamase
B-lactamases are heterogeneous bacterial enzymes which cleave b-lactam ring of penicillins and cephalosporins to inactive the drugs. Many bacterial species are known to produce them.
Clinically the most important bill at time is producer are staphylococcus aureus, haemophilus species and Neisseria gonorrheae.
The enzyme may be located on the chromosome but more often are on Plasmids which are transferable from one bacterium to another.
The B-lactamases in some bacteria is constitutive, i.e., present even in the absence of the antibiotic; or inducible i.e. produced only in the presence of the antibiotic. Some B-lactamases effect many antibiotics other acts either on penicillin or cephalosporins.
Test for the detection of b-lactamase
There are three types of test in used for the directions of b-lactamase; acid formation method, ideometric method and nitrocefin test.
Staphylococci produce B-lactamase only when exposed to penicillins. The resistance to penicillin G and related compounds by staphylococci is due to this b-lactamase. However, this enzyme are not active against the penicillinase-resistant penicillins such as methicilins and against cephalosporins.
Penicillin resistant staphylococci may produce a zone of inhibition if the small quantity of the penicillinase produced by these staphylococci is not sufficient to inactivate the high-level of penicillins near the disc. This zone show a characteristic heaped-up clearly-defined edge without any gradually fading growth, as can be seen in sensitive strains. All the strain which was show such a heaped of edge should therefore be regarded as resistant to penicillin.
Twenty to 40% of type B haemophilus influenzae produce b-lactamase, and are resistant to penicillin and ampicillin. Treatment with combination of amoxicillin and clavulanic acid (augmentin) is effective. Clavulanic acid is an inhibitor of many b-lactamase. It is recommended that haemophilus influenzae isolation from any severe infection should be tested for b-lactamase production.
A strain of H influenzae are resistant to ampicillin by mechanisms other than b lactamase activity. Therefore, a B-lactamase negative strain should also be tested against ampicillin before using the job for therapy. With 2 microgram ampicillin disc, a zone of 20 mm or more is considered susceptible and a lesser-known resistant.
Perior to 1975, penicillin was considered the drug of choice for the treatment of uncomplicated gonococcal infections.
However, most N. gonorrheae have acquired resistance to penicillin by plasmid-mediated penicillinase production.
This penicillinase producing N. gonorrheae (PPNG) can be detected by the B-lactamases test. Cefftriaxone is effective against PPNG.