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Infectious diseases continue to impose a huge global public health burden, accounting for more than one-quarter of all deaths annually and a similar fraction of morbidity. In addition to long-established infections such as malaria, tuberculosis, and measles, we are also seeing the frequent emergence of new infectious diseases: HIV/ AIDS was unknown 25 years ago and is now one of the biggest killers; the virus responsible for severe acquired respiratory syndrome (SARS) caused a minor public health problem but still had severe economic repercussions for affected countries; and there is considerable concern about the possible emergence of a new subtype of influenza A, with potentially devastating, global consequences.

Pulsed-field gel electrophoresis (PFGE) was used to investigate the dissemination and diversity of ampicillin-resistant (Amp r ) and nalidixic acid-resistant (Nal r ) commensal Escherichia coli strains in a cohort of 48 newborn calves. Calves were sampled weekly from birth for up to 21 weeks and a single resistant isolate selected from positive samples for genotyping and further phenotypic characterization. The Amp r population showed the greatest diversity, with a total of 56 different genotype patterns identified, of which 5 predominated, while the Nal r population appeared to be largely clonal, with over 97% of isolates belonging to just two different PFGE patterns. Distinct temporal trends were identified in the distribution of several Amp r genotypes across the cohort, with certain patterns predominating at different points in the study. Cumulative recognition of new Amp r genotypes within the cohort was biphasic, with a turning point coinciding with the housing of the cohort midway through the study, suggesting that colonizing strains were from an environmental source on the farm. Multiply resistant isolates dominated the collection, with >95% of isolates showing resistance to at least two additional antimicrobials. Carriage of resistance to streptomycin, sulfamethoxazole, and tetracycline was the most common combination, found across several different genotypes, suggesting the possible spread of a common resistance element across multiple strains. The proportion of Amp r isolates carrying sulfamethoxazole resistance increased significantly over the study period ( P < 0.05), coinciding with a decline in the most common genotype pattern. These data indicate that calves were colonized by a succession of multiply resistant strains, with a probable environmental source, that disseminated through the cohort over time.

The presence of ampicillin-resistant Escherichia coli (Amp r E. coli ) in the fecal flora of calves was monitored on a monthly basis in seven cohorts of calves. Calves were rapidly colonized by Amp r E. coli , with peak prevalence in cohort calves observed in the 4 months after the calves were born. The prevalence of calves yielding Amp r E. coli in cohorts consistently declined to low levels with increasing age of the calves ( P < 0.001).

I would like to comment on the excellent article by Mark E. J. Woolhouse entitled “Where Do Emerging Pathogens Come From?—Understanding the Origins of Pathogens will Help us to Combat Emerging Infectious Diseases” (Microbe, November 2006, p. 511–515). The author described that various factors play a role in the emergence and reemergence of pathogens, including the diversity in animal reservoirs, species jumps, factors relating to the host, pathogen, and the wider environment, and last but not least, human activity. This clearly shows that the emergence and reemergence of pathogens is quite a complex process, one we perhaps have only just begun to understand in more detail.