An infant with aplastic anaemia
5.1. An infant with aplastic anaemia
Tenover and colleagues (Rasheed et al., 1997) studied serial isolates, obtained over a 3-month period, from the blood of an infant with idiopathic aplastic anaemia. Extensive broad spectrum antibiotic treatment took place during this period to treat fever episodes, in parallel with treatment for the aplastic anaemia. The first confirmed E. coli isolate from blood was some ⬃10 weeks after initial admission. Eight days after the initial isolate, three dis- tinct phenotypes were noted among the isolates from the blood on the same day suggesting that the population was diverging or that the infected locus contained an already diverged community. The most distinctive isolate had acquired high-level resistance to tetracycline, gentamicin, and sulfonamides. This strain was found to have a modified plasmid banding pattern consistent with the possible enlargement of a preexisting 120 kDa plasmid. The multiple drug resistance phenotypes could be transferred by conjugation, whereas the -lactamases could not (Rasheed et al., 1997). Midway through this period, the resistance spectrum of the isolates changed dramatically with the acquisi- tion of high-level resistance to several oxyimino cephalosporins, for example, cefotaxime and ceftazidime. PCR analysis coupled with IEF/nitrocefin-based detection of -lactamases showed that the new variants had acquired an SHV -lactamase in addition to the TEM-type enzyme already present. Over the time course of the study, strains lacking the porin OmpF were also detected, although the final pair of isolates exhibited some OmpF protein. As indicated above, analysis of the genome by pulsed field gel electrophoresis (PFGE) and by arbitrarily-primed PCR showed that all of the strains originated from a sin- gle clone. Thus, there was considerable diversity selected in the patient over a relatively short time period
The nature of this patient’s illness suggests that the major source of the infecting organisms was the intestine. Thus, there are several parallel processes that may be responsible for the evolution of the antibiotic resistance. The intestinal flora may have contained a group of related strains that were inter- acting to effect the transfer of drug resistant determinants, for example, the SHV -lactamase. The OmpF ⫺ strains may have been resident the whole time and were then selected by changes in the antibiotic regime. Alternatively, mutations may have arisen during the course of treatment. It is not clear from the study if the strains were mutators. The reappearance of the porin- containing strains among the final blood isolates suggests the potential for coexistence in the gut of both OmpF-lacking and “normal” strains and thus both
Evolution of Antibiotic Resistance within Patients 379 might have been the focus of evolution throughout the period of observation.
Data from the author’s group suggest that evolution of a related group of
E. coli isolates in the gut is probably a frequent occurrence. Studies in the author’s laboratory have analysed the complexity of the faecal E. coli in the gut of an elderly patient exhibiting bacteraemia (J. Park,
F. MacKenzie, I. M. Gould, and I. R. Booth, unpublished data). A remarkable diversity was observed below the molecular level. PFGE and single-gene RFLP patterns suggested that there were just two major clones, one of which was also isolated from the patient’s blood. The minor clone, which was repre- sented by 3/20 faecal isolates, exhibited a completely different PFGE pattern,
a unique ompC gene sequence, had acquired sucrose metabolism and a very high MIC for amoxicillin. The pattern of plasmids in this strain was also quite distinct from the major clone, although by transformation experiments it was demonstrated that they also had at least one plasmid in common (J. Park,
F. MacKenzie, I. M. Gould, and I. R. Booth, unpublished data). However, the major faecal clone, which was identical to the blood isolate, could be subdi- vided into two metabolic types with respect to the rate of metabolism of melibiose. In addition, the isolates exhibited distinct MIC values for amoxi- cillin and differed in the degree of inhibition of the -lactamases by clavulanic acid. While not an exhaustive analysis, the data point to considerable diversity coexisting among the resident E. coli flora in a single patient (J. Park,
F. MacKenzie, I. M. Gould, and I. R. Booth, unpublished data). Other studies have concluded that E. coli is the dominant carrier of antibiotic resistance in the faecal flora and have suggested that the main potential route of transmis- sion is by conjugation (Osterblad et al., 2000). In this study, comparisons were made between isolates from the community, from short-stay patients and from long-stay (geriatric) patients. In all three groups, E. coli was the main resistant organism, but it was notable that some of the hospital samples (short-stay) had increased levels of multidrug resistant Enterobacteriaceae that exhibited distinct profiles from the multidrug resistant E. coli.