A susceptibility test may be performed in the clinical laboratory for two main purposes:
• to guide the clinician in selecting the best antimicrobial agent for an individual patient;
• to accumulate epidemiological information on the resistance of microorganisms of public health importance within the community.
Susceptibility tests as a guide for treatment
Susceptibility tests should never be performed on contaminants or commensals belonging to the normal flora, or on other organisms that have no causal relationship to the infectious process. For example, the presence of Escherichia coli in the urine in less than significant numbers is not to be regarded as causing infection, and it would be useless and even misleading to perform an antibiogram.
Susceptibility tests should be carried out only on pure cultures of organisms considered to be causing the infectious process. The organisms should also be identified, as not every microorganism isolated from a patient with an infection requires an antibiogram.
Routine susceptibility tests are not indicated in the following situations:
• When the causative organism belongs to a species with predictable susceptibility to specific drugs. This is the case for Streptococcus pyogenes and Neisseria meningitidis, which are still generally susceptible to penicillin. (However, there have recently been a few reports of sporadic occurrences of penicillin-resistant meningococci.) It is also the case for faecal streptococci (enterococci), which, with few exceptions, are susceptible to ampicillin. If resistance of these microorganisms is suspected on clinical grounds, representative strains should be submitted to a competent reference laboratory.
• If the causative organism is slow-growing or fastidious and requires enriched media, e.g., Haemophilus influenzae and Neisseria gonorrhoeae, disc-diffusion susceptibility tests may give unreliable results.
The emergence of β-lactamase-producing variants of these species has led to the introduction of special tests, such as the in vitro test for β-lactamase production. It will be the responsibility of the central and regional laboratories to monitor the susceptibility of pneumococci, gonococci, and Haemophilus. If problems arise with resistant strains, the peripheral laboratories should be alerted and instructions should be given on appropriate testing methods and on alternative treatment schemes.
• In uncomplicated intestinal infections caused by salmonellae (other than S. typhi or S.
paratyphi), susceptibility tests are not routinely needed. Antibiotic treatment of such infections is not justified, even with drugs showing in vitro activity. There is now ample evidence that antimicrobial treatment of common salmonella gastroenteritis (and indeed of most types of diarrhoeal disease of unknown etiology) is of no clinical benefit to the patient. Paradoxically, antibiotics prolong the excretion and dissemination of salmonellae and may lead to the selection of resistant variants.
Susceptibility tests as an epidemiological tool
Routine susceptibility tests on major pathogens (S.
typhi, shigellae) are useful as part of a comprehensive programme of surveillance of enteric infections. They are essential for informing the physician of the emergence of resistant strains (chloramphenicol-resistant S. typhi, co-trimoxazole-resistant and ampicillin-resistant shigellae) and of the need to modify standard treatment schemes. Although susceptibility testing of non-typhoid salmonellae serotypes causing intestinal infection is not relevant for treating the patient, the appearance of multiresistant strains is a warning to the physician of the overuse and misuse of antimicrobial drugs.
Continued surveillance of the results of routine susceptibility tests is an excellent source of information on the prevalence of resistant staphylococci and Gram-negative bacilli that may be responsible for cross-infections in the hospital. Periodic reporting of the susceptibility pattern of the prevalent strains is an invaluable aid to forming a sound policy on antibiotic usage in the hospital by restriction and/or rotation of life-saving drugs, such as the aminoglycosides and cephalosporins.