Instrumentation for antimicrobic susceptibility testing: yesterday, today, and tomorrow

Laboratory tests used to guide antimicrobial therapy

Laboratory tests that are usually considered helpful in guiding antimicrobial therapy include antimicrobial agent susceptibility tests, determination of bacterial production of beta-lactamase, and assay of specific antimicrobial levels in serum and other body fluids. Susceptibility tests should be performed primarily on clinically significant isolates from critical specimens (such as blood or other normally sterile body fluids or tissues) with use of standardized methods established by the National Committee for Clinical Laboratory Standards. Reporting of results should be selective so that clinicians are encouraged to use the least expensive but useful agent in a group--for example, first-generation rather than third-generation cephalosporins. Because standardized methods are not available for assays of serum inhibitory and bactericidal activity, the accuracy and clinical utility of these tests are as yet undetermined. Determination of bacterial resistance to antimicrobial agents is the most important goal of susceptibility testing. Special methods are needed to detect methicillin-resistant staphylococci, high-level aminoglycoside- and glycopeptide-resistant enterococci, and antimicrobial-resistant strains of Neisseria gonorrhoeae, Streptococcus pneumoniae, and Haemophilus influenzae. Accurate measurement of serum concentrations of antimicrobial agents is important to ensure that therapeutic levels have been obtained and to avoid excessive levels of potentially toxic agents such as aminoglycosides and glycopeptides, especially when renal function is compromised.

Application of impediometry to rapid assessment of liquid culture media

The impedance method provides as unique opportunity to determine microbial activity and kinetics. Since the metabolic processes depend on the nature and quality of the culture medium, impediometry allows the assessment of liquid culture media. Impedance microbiology represents an approach to quantitative microbiology. We investigated the influence of pH, composition and variation of the amounts of industrially made dry media, overheating during the dissolving or sterilisation processes, and qualitative differences between batches of the same culture medium. Using glucose broth as an example, we showed that impediometry allows quantitative, microbial assessment of culture media. Inaccurate preparation of the culture medium could be detected quickly by the use of impediometry. The method is very simple to perform, requires no sample preparation, allows rapid assessment of liquid culture media, and interprets results automatically with the aid of a microcomputer.