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close this bookBasic Laboratory Procedures in Clinical Bacteriology (WHO; 1991; 128 pages)
View the documentPreface
View the documentIntroduction
open this folder and view contentsQuality assurance in microbiology
close this folderPart I. Bacteriological investigations
open this folder and view contentsBlood
open this folder and view contentsCerebrospinal fluid
open this folder and view contentsUrine
open this folder and view contentsStool
open this folder and view contentsLower respiratory tract infections
open this folder and view contentsUpper respiratory tract infections
open this folder and view contentsSexually transmitted diseases
close this folderPurulent exudates, wounds, and abscesses
View the documentIntroduction
View the documentCommonly encountered clinical conditions and the most frequent etiological agents
View the documentCollection and transportation of specimens
View the documentMacroscopic evaluation
View the documentMicroscopic examination
View the documentCulture
View the documentIdentification
View the documentSusceptibility testing
open this folder and view contentsAnaerobic bacteriology
open this folder and view contentsAntimicrobial susceptibility testing
open this folder and view contentsPart II. Essential media and reagents for isolation and identification of clinical pathogens
View the documentSelected further reading
View the documentSelected WHO publications of related interest
View the documentBack Cover

Susceptibility testing

Antibiotics may not always be needed for the management of patients with wounds, abscesses, or exudates. Proper surgical incision, drainage and débridement are generally more important than antibacterial drugs. The results of susceptibility tests should be made available, however, within 48 hours after receiving the specimen.

Routine susceptibility tests should not be performed on bacteria that have a known sensitivity pattern, such as streptococci, Pasteurella, and Actinomyces, which are almost without exception susceptible to penicillins.

For Enterobacteriaceae, non-fermentative Gram-negative rods, and staphylococci, the standardized disc-diffusion test (Kirby-Bauer) should be used. Only antibiotics currently being used by the requesting physicians should be tested. New and expensive antibiotics should only be tested (or reported) on special request, or when the isolate is resistant to other drugs.

Problems are often encountered when testing the susceptibility of staphylococci, both S. aureus and S. epidermidis. Over 80% of isolates, even from the community, produce β-lactamase and are resistant to penicillin and ampicillin. Infections caused by penicillin-resistant staphylococci are often treated with β-lactamase-resistant penicillins of the meticillin-group (oxacillin, cloxacillin, etc.). The oxacillin disc (1 μg) is currently recommended for testing the susceptibility to this group. Oxacillin discs are stable, and apt to detect resistance to the whole group. This resistance is often of the heteroresistant type, i.e., it involves only a part of the bacterial population. As heteroresistance of staphylococci is easier to recognize at low temperatures, the incubator temperature should not exceed 35 °C. A heteroresistant strain shows, within an otherwise definite zone of inhibition, a film of hazy growth, or numerous small colonies, that are often dismissed as contaminants. If such growth appears, a Gram-stained smear is indicated to exclude contamination.

Heteroresistant strains are clinically resistant to all the cephalosporins, as well as to the meticillin group. For this reason staphylococci need not be tested for susceptibility to cephalosporins. There is complete cross-resistance between penicillin and ampicillin. Staphylococcal susceptibility to ampicillin should therefore not be tested separately.

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