β-Lactam resistance and β-lactamase expression in clinical Stenotrophomonas maltophilia isolates having defined phylogenetic relationships

Aims: To test the hypothesis that Stenotrophomonas maltophilia isolates from certain phylogenetic groups have predictable beta-lactamase expression and beta-lactam resistance profiles. Methods: Isolates were grouped using sequences of the 16S rRNA gene and smeT-smeD intergenic region. beta-Lactamase activities in cell extracts were quantified spectrophotometrically and beta-lactam MICs were determined using agar dilution methodology and Etest as appropriate. Results: A collection of 50 clinical S. maltophilia isolates from Europe and North, South and Central America were phylogenetically grouped. Group 'A' (22 out of 50) includes remarkably genetically homogeneous isolates; group 'B' (17 out of 50) includes isolates that are genetically heterogeneous and quite distinct from those of group A. Members of these two groups are, however, indistinguishable in terms of their beta-lactam resistance and beta-lactamase expression phenotypes. In contrast, isolates from group 'C', which are less common (8 out of 50), are considerably more susceptible to beta-lactams owing to reduced inducibility of beta-lactamase expression following beta-lactam challenge. Conclusions: The majority of S. maltophilia clinical isolates behave similarly in terms of beta-lactamase expression and beta-lactam resistance properties, despite considerable phylogenetic variability.