Screening for β-lactam resistance by penicillin G in the Streptococcus anginosus group challenged by rare strains with altered PBPs.

BACKGROUND

Streptococcus anginosus group (SAG) strains are common pathogens causing abscesses and bacteraemia. They are generally susceptible to β-lactams, which constitute first-line treatment. EUCAST recommends testing penicillin G susceptibility to screen for β-lactam resistance. Isolates categorized as susceptible (negative screening) can be reported as susceptible to aminopenicillins and third-generation cephalosporins.

OBJECTIVES

To assess the reliability of penicillin G resistance screening in predicting β-lactam resistance in SAG blood culture isolates, and to investigate isolates for which this test would be unreliable.

METHODS

We determined the susceptibility to penicillin G, amoxicillin and ceftriaxone of 90 SAG blood culture isolates, all with negative penicillin G resistance screening. β-Lactam-resistant strains were sequenced and compared with susceptible reference SAG strains.

RESULTS

We detected two isolates displaying β-lactam resistance, especially to third-generation cephalosporins, despite negative screening for penicillin G resistance. For these isolates, amino acid substitutions were identified next to the essential PBP motifs SxxK, SxN and/or KS/TGS/T. Changes in these motifs have been previously linked to β-lactam resistance in Streptococcus pneumoniae.

CONCLUSIONS

Our study suggests that aminopenicillin and third-generation cephalosporin susceptibility should be determined for SAG strains in the event of severe infection as screening for penicillin G resistance might not be sufficient to detect resistance mechanisms that predominantly affect cephalosporins. The PBP sequencing of resistant SAG strains allowed us to detect amino acid changes potentially linked to β-lactam resistance.