Two factors create confusion in clearly defining multidrug resistant S. typhia (MORST). (1) Extensive clinical experience over many years has established in vivo efficacy of chloram-phenicol (Cm), ampicillin (Am)and trimethoprim-sulfamethoxazoles (TMP-SMZ) in the treatment of typhoid fever, and this correlates accurately with the in vitro pattern of susceptibility. In addition, clinical microbiology laboratories routinely test susceptibility of S. typhia to aminoglycosides, first generation cephalosporins tetracyclines, furazolidine and nalidixic acid. However, none of these antibiotics has ever been shown to be consistently effective in the treatment of typhoid fever regardless of in vitro susceptibility. Thus data regarding these drugs are of no clinical relevance in making therapeutic decisions. However, these agents may serve as resistance markers ot S. typhia strains because resistance to them is commonly linked with resistance to first line antibiotics such as Cm, Am and TMP SMZ. (2) Epidemiologic studies define MDRST as strains resistant to two or more antibiotics in vitro, even if the antibiotics tested are known to be ineffective clinically. AS. typhia strain with in vitro resistance to furazolidine gentamicin and cephalothin but susceptible to Cm, Am and TMP SMZ may be classified as multidrug-resistant from an epidemiologic viewpoint. However, such resistance is irrelevant from a clinical perspective because the organism is susceptible to all potentially useful first line antibiotics.

Therefore the term MDRST should denote only those strains for which the resistance pattern is clinically relevant and include all three of the first line antibiotics.

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