Escherichia coli and other Enterobacteriaceae, which produce an extended-spectrum beta-lactamase (ESBL), have become the subject of many studies during recent years in both human and veterinary medicine. ESBLs confer resistance to aminopenicillins, cephalosporins (including those of the 3^rd and 4^th generation) and monobactams. As beta-lactams are currently the most frequently sold class of antimicrobial agents in veterinary medicine in Germany, the presence of ESBL genes limits the therapeutic options.

In E. coli, there is not a single ESBL gene but numerous ESBL genes which belong to various gene classes. Among them, ESBL genes of the CTX-M, SHV, TEM and OXA classes are most widespread. Class- or even subclass-specific PCR assays can be used for a first characterization. However, the correct identification of a specific ESBL gene requires the determination of its complete sequence from start codon to stop codon. ESBL genes can be located in the chromosomal DNA or on plasmids. Either of these locations has implications with regard to the spread of the respective ESBL gene. When located chromosomally, vertical transfer during cell division is most likely. For tracing the dissemination of such ESBL-producing isolates, various typing methods can be applied. PCR-directed phylotyping classifies E. coli isolates in different phylogroups. Multi-locus sequence typing is not suitable for short-term epidemiological analyses (e.g. outbreak investigations), but for phylogenetic analyses and long-term epidemiological studies. In contrast, pulsed-field gel electrophoresis (PFGE) is most suitable to compare isolates in short-term epidemiological studies. DNA microarrays will help to identify the virulence and resistance gene content of E. coli isolates. Next generation sequencing, followed by bioinformatic analyses, will surely be used more extensively in the future for outbreak investigations. Plasmid analysis, including S1-PFGE, restriction analysis, PCR-based replicon typing, and finally whole plasmid sequence analysis, is important to follow the dissemination of an ESBL gene-carrying plasmid between isolates, species and genera. Examples of ESBL-producing E. coli will be provided to illustrate the use of the different methods.