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Human listeriosis causes severe disease and, when invasive, leads to high fatality rates. Clinical spectrum is varied with septicemia being more common, followed by meningoencephalitis in immunocompromised individuals and elderly.[1],[2],[3]Listeria monocytogenes (Lm) has a higher misdiagnosis rate with fast progression and poor prognosis.[4],[5] Conventional culture method using blood or cerebrospinal fluid is time-consuming and has very low positive detection rate.[6] Despite adequate antibiotic treatment, the overall mortality is still high (25%–30%) and neurological sequelae are frequent. In this short report, we describe three clinical cases of Lm identified using conventional method; subsequently, the strains were characterized using next-generation sequencing. The clinical and microbiological characteristics of the isolates of the three patients are summarized in [Table 1]. All the three patients had risk factors such as type 2 diabetes mellitus, systemic hypertension, chronic kidney disease stage 5, lung fibrosis, and malignancy (germ cell tumor), which leads to impaired cell-mediated immunity, further predisposing to severe infection. ResFinder revealed only fosX gene in all three genomes. Accordingly, the study isolates were resistant to cephalosporins and susceptible to ampicillin, penicillin, and SXT (Trimethoprim/Sulphamethoxazole). Very few compounds have a bactericidal effect on Lm cells. Ampicillin or penicillin G was reported to be the best treatment options for listeriosis based on their bacteriostatic effect. The combination of AMP/PEN in combination with gentamicin will enhance the bactericidal effect of the therapy.[7]
Table 1: Clinical and microbiological characteristics of listeria isolatesIntrinsic resistance to cephalosporins was reported to be mediated by oatA mutations and PBP3 mutations. In the study isolates, the site of insertions and non-sense mutation in oatA were in S96A, D440E, after 433 DK insertion, after 442 DSKE insertion, A443T, S445A, S449N, E450G, K452N, E453Q, K456V, S463T, I470M, D539E, A576S, and A607S. Regions of mutations in PBP3 were G66S, N98D, D171E, A223D, P236A, T580A, K584A, E608Q, and I650V [Figure 1]. OatA mutants were known to induce early secretion of proinflammatory cytokines and chemokines in vivo, which recites the importance of oatA in limiting innate immune responses, thereby promoting bacterial survival in the host.[8] This insertional inactivation of lmo0441 and lmo2229 (coding PBP3 and PBPA2, respectively) greatly reduces the intrinsic resistance of Lm to cephalosporins.[9] The patient B, who had a favorable outcome, was initiated on ampicillin with gentamicin following the culture reports, hence showed clinical recovery. The patient C was initiated on carbapenem, which did not modify the disease course and led to adverse outcomes. In this study, insertions and mutations reported in oatA and PBP3 gene were known to be the primary mechanism of resistance to higher generation cephalosporins. This study adds to the existing evidence on the resistance mechanisms of Lm and emphasises the importance of microbiological diagnosis, and also indicates the significance of considering the host predisposition before initiating empirical therapy with higher antibiotics such as cephalosporins or carbapenems.
Figure 1: Protein multialignment showing insertions and mutations in oatA geneResearch quality and ethics statement
The authors followed applicable EQUATOR Network (https://www.equator-network.org/) guidelines, notably the CARE guideline, during the conduct of this report.
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References 
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CheckDOI: 10.4103/jgid.jgid_94_22
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