Gnotobiotic piglet infection model for evaluating the safe use of antibiotics against Escherichia coli O157:H7 infection

In humans, especially children, infection with Shiga toxin (Stx)–producing Escherichia coli (STEC) is strongly associated with the development of hemolytic uremic syndrome (HUS), the leading cause of kidney failure in children [1–4] and acute encephalopathy [5–7]. In the United States, the most significant Stx-producing organisms are enterohemorrhagic E. coli (EHEC) of serotype O157 [8–11]. STEC strains produce 2 subclasses of Stx, Stx1 and Stx2 [12, 13]. Although similar in basic structure, binding specificity, and mode of action, the 2 toxins are immunologically distinct and are regulated in different ways [14]. There is now strong epidemiological evidence showing that HUS development is more closely associated with Stx2-producing strains than with Stx1-producing strains or strains that produce both toxins [15]. This association is further strengthened by studies in gnotobiotic piglets infected with isogenic strains producing Stx2, Stx1, or both: the development of systemic complications as the indicators of HUS was associated with the production of Stx2, and the greatest rate of development was associated with Stx2 production alone [16]. Currently there are no recommended treatment regimens to lessen the risk of HUS in patients [5, 17–19], and the available treatment regimens need to be reevaluated in an animal model. The normal first-line clinical treatment of an enteric infection, the administration of appropriate antibiotics, is highly controversial. Some investigators have concluded that the administration of antibiotics has no effect on the risk of HUS development [20–22], and some have even concluded that the risk is lessened after fosfomycin treatment [23]. Two epidemiological studies—a national surveillance study conducted by the Centers for Disease Control and Prevention and a case-control study involved in a network of participating centers in Washington, Idaho, Oregon, and Montana—found that antibiotic therapy for STEC enteritis significantly increased the risk of HUS [24, 25]. A recent review indicated that the rate of HUS development after O157:H7 infection may increase from 15% to 50% after antibiotic use [19]. The effects of antibiotics on Stx1 and Stx2 production and/or release may explain a possible antibiotic-related increase in HUS development. The genes for the toxins are usually carried on lysogenic phages. Adverse conditions often cause the induction of these temperate phages, and, as the phage replicates within the bacterium, large quantities of toxin are produced. It has been shown in vitro that treatment with certain antibiotics causes phage induction and subsequent increases in toxin levels [26]. To correlate in vitro results with in vivo treatment regimens of currently licensed antibiotics or to evaluate a novel class of antibiotics or medications before their clinical application requires a suitable animal model of infection in which detrimental and beneficial antibiotics can be reliably tested. We reevaluated the effects of several antibiotics on toxin expression in vitro. For one antibiotic, ciprofloxacin, the mechanism of enhancing Stx2 production via phage induction has been validated. We report the development of an animal model system to allow for the testing of potentially beneficial and deleterious antibiotics. On the basis of our in vitro results and the treatment regimens for children, we evaluated the efficacy of antibiotics (ciprofloxacin and azithromycin) against E. coli O157:H7 infection in our animal model.