Public Health England (PHE) has issued new testing and treatment guidance relating to a multi-drug resistant Shigella sonnei cluster (CTX-M-27), which was identified in England, Wales, and Scotland during 2018.
This strain is of concern due to its reduced susceptibility to fluoroquinolones and the macrolide resistance markers erm(B) and mph(A).
Treatment of infections caused by this strain with first-line agents like quinolones, azithromycin, and ceftriaxone may not be effective due to its multi-resistance, cautioned PHE.
Microbiologists should be aware that this strain of S. sonnei is phenotypically an extended spectrum β-lactamase (ESBL) producer, resistant to amoxicillin, co-amoxiclav, ceftriaxone, ceftazidime, and co-trimoxazole, noted PHE.
The azithromycin minimum inhibitory concentrations (MICs) have been >256 mg/L (epidemiological breakpoint of 16 mg/L) in all except 3/17 strains. It is consistently susceptible to chloramphenicol, ertapenem, temocillin, gentamicin, colistin, mecillinam, and fosfomycin.
Laboratories should follow European Committee on Antimicrobial Susceptibility Testing (EUCAST) protocols for susceptibility testing of Enterobacterales with these antibiotics, said PHE.
ESBL-producing S. sonnei can cause severe infection in men who have sex with men (MSM) and treatment may be required in some cases that have prolonged dysentery or sepsis.
Oral treatment options for this strain are limited to antibiotics such as chloramphenicol, mecillinam and fosfomycin.
Use of either mecillinam or fosfomycin would be off-label or unlicensed, so should only be considered for treating uncomplicated cases such as prolonged diarrhoea, according to PHE.
Due to a lack of evidence of their efficacy in severe infections, mecillinam and fosfomycin should not be used in the immunocompromised or cases with sepsis or severe colitis; consideration should be given to intravenous agents like ertapenem or temocillin, the guidance says. The full guidance document is available here.