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Objective of Beta (β)-Lactamase Test
- To detect the enzyme beta-lactamase, which confers penicillin resistance to various bacterial organisms.
Principle of Beta (β)-Lactamase Test
Various bacteria produce a class of enzymes called beta-lactamases, which may be mediated by genes on plasmids or chromosomes. Production of beta-lactamase may be constitutive or induced by exposure to antimicrobials. Beta-lactamases hydrolyze (and thereby inactivate) the beta-lactam rings of a variety of susceptible penicillins and cephalosporins. Beta-Lactamase Test rapidly detects the presence of beta-lactamase enzyme produced by strains of Staphylococcus aureus, Neisseria gonorrhoeae, Branhamella catarrhalis and Haemophilus influenzae. These enzymes confer resistance to a number of penicillin antibiotics by cleaving the beta-lactam ring of penicillins and cephalosporin antibiotics, resulting in inactivation of these drugs. This mode of action forms the basis of the beta-Lactamase test reaction.
Rapid beta-lactamase tests can yield clinically relevant information earlier than an MIC or disk diffusion test. Several clinical tests have been devised to detect beta-lactamases. These tests include the iodometric method, the acidometric method, and chromogenic substrates. Iodometric methods are suitable for testing N. gonorrhoeae. Acidometric methods produce acceptable results with Haemophilus spp., N. gonorrhoeae and staphylococci. Nitrocefin has the wide spectrum of susceptibility and sensitivity of the commercially available beta lactams. Nitrocef Disks are impregnated with nitrocefin, a chromogenic cephalosporin. As the amide bond in a beta-lactam ring is hydrolyzed by a beta-lactamase, nitrocefin changes color from yellow to red. Bacteria which produce beta-lactamase in significant amounts produce this yellow to red color change on the Nitrocef Disk. These beta-lactamases are capable of inactivating “penicillinase-labile-penicillins”, such as amoxicillin, ampicillin, penicillin, carbenicillin, mezlocillin, and piperacillin.
Procedure of Beta (β)-Lactamase Test
- Using a single disk dispenser, dispense the disk from the cartridge into an empty petri dish or onto a microscope slide.
- Moisten disc with 1 drop of sterile distilled water.
- With a sterilized loop or applicator stick remove several well-isolated similar colonies and smear onto a disk surface.
- Observe disk for colour change.
Result Interpretation of Beta (β)-Lactamase Test
Positive reaction: yellow to red colour change on the area where the culture is applied.
Note: For most bacterial strains a positive result will develop within 5 minutes. However, positive reactions for some staphylococci may take up to 1 hour to develop and colour change does not usually develop over the entire disk.
Negative reaction: no colour change on the disc
|Organisms||Result||Approximate reaction time||Interpretation|
|Staphylococcus aureus||Positive||1 hour||Resistant to penicillin, ampicillin, carbenicillin. Probably susceptible to cephalothin, methicillin, oxacillin, naficillin and other penicillinaseresistant penicillins.|
|Enterococcus faecalis||Positive||5 min||Resistant to penicillin and ampicillin|
|Hameophilus influenzae||Positive||1 min||Resistant to ampicillin Susceptible to cephalosporins|
|Neisseria gonorrhoeae and Branhamella catarrhalis||Positive||1 min||Resistant to penicillin|
Limitations of Beta (β)-Lactamase Test
- Beta-lactamase detection with the Nitrocef Disk should not entirely replace conventional susceptibility test methods, as other factors also influence the results of such tests, and on occasion intrinsic resistance to beta-lactam antimicrobials has not been correlated with production of beta-lactamase.
- Do not over saturate the tip as it could dilute the reagent.
- Detection of beta-lactamase activity in staphylococci may take up to one hour. Induction of the enzyme may also be required, this can be done by testing growth from the zone margin around an oxacillin disk.
- A negative result does not rule out resistance due to other mechanisms.
- Nitrocef Disk method cannot be used to test members of Enterobacteriaceae, Pseudomonasspecies or other aerobic, gram-negative bacilli because the results may not be predictive of susceptibility to the beta-lactams most often used for therapy.
- The Nitrocef Disk cannot be used for organisms where penicillin resistance is not due to beta-lactamase production, such as Streptococcus pneumoniaeand viridans streptococci.
Quality Control of Beta (β)-Lactamase Test
Staphylococcus aureus (ATCC 43300): Positive
Haemophilus influenzae (ATCC 33533): Positive
Branhamella catarrhalis (ATCC 25240): Negative