Metallo-β-Lactamase and Extended-Spectrum-β-Lactamase Production by Serratia Strains [Letter]

Dear editor

In a recent study,¹ thirty-six Serratia marcescens clonally disseminated isolates harbouring a blaKPC-2 carrying plasmid were reported from a Zhejiang University School of Medicine hospital. We felt it appropriate to share our observations on Serratia strains in the last three years isolated from referred veterinary and medical clinical cases. In our laboratory, none of the Serratia marcescens isolates from samples referred from veterinary and medical clinical cases and, fish for meat, milk and environmental samples at Indian Veterinary Research Institute, India either produced metallo-β-lactamase (MBL) or was resistant to any of the three carbapenems (ertapenem, meropenem and imipenem) drugs tested. However, out of 75 isolates of Serratia species (Table 1) identified and characterised using growth and biochemical characteristics,^2,3 13 (17.3%) isolates produced MBL and 39 (52%) produced extended-spectrum-β-lactamase (ESBL) on testing with E-strip assay using MBL and ESBL strips procured from BioMerieux, Marcy-l’Étoile, France, using the standard method prescribed by CLSI.⁴ In our observations, none of the eight Serratia marcescens had imipenem or meropenem MIC >2 µg mL⁻¹ but six of those produced ESBL. The 13 Serratia strains producing MBL belonged to S. ficaria (2), S. liquefaciens (1), S. odorifera (6) and S. plymuthica (4). The MIC of Serratia strains positive for MBL had imipenem MIC ≥24 µg mL⁻¹. Six of the 13 MBL positive isolates were from buffalo milk (S. ficaria 2, S. odorifera 4) collected from different dairy farms indicating no clonal spread which was further evidenced by their different antibiograms (data not shown). The similar non-clonal spread was also suspected for rest of the 7 isolates of Serratia producing MBL. However, two Serratia rubidaea isolates from dogs having surgical wound infection getting operated in the same facility matched for their antibiogram and produced ESBL indicating nosocomial infection with the same clone.

Table 1 Metallo-β-Lactamase (MBL) and Extended-Spectrum-β-Lactamase (ESBL) Producing Serratia Species Strains Isolated at Clinical Epidemiology Laboratory, ICAR-Indian Veterinary Research Institute, Izatnagar, India

Serratia strains in our study were associated with septicaemia leading to death in eight cases, abortion in cattle, wound infection and dysentery in horses (Table 1) indicating their potential to cause a variety of ailments as observed in humans.¹ Though Serratia strains rarely cause infection in animals and human beings, isolation of MBL and ESBL producing Serratia strains from clinical samples is not a novelty and such strains have often been reported earlier.^1,5-7 Detection of ESBL and MBL producing Serratia strains in food (fish and milk) and environmental (Neem, Azadirachta indica, leaves, chewed raw in the morning by many Indians to boost their health) samples pose a public health threat anytime if such potentially pathogenic strains started to propagate and spread clonally as reported in the recent study.¹ Looking at the profile of Serratia strains, it is need of the day that isolation of a Serratia from clinically samples should not be ignored even if it is not belonging to S. marcescens as in our observations only four of the 44 isolates from clinical samples belonged S. marcescens species and 40 were of other species.