Does the presence of multiple β-lactamases in Gram-negative bacilli impact the results of antimicrobial susceptibility tests and extended-spectrum β-lactamase and carbapenemase confirmation methods?

Investigating the validity of mCIM and sCIM phenotypic methods in screening Pseudomonas aeruginosa isolates producing IMP, VIM, and NDM metallo-beta-lactamases isolated from burn wounds

Metallo-beta-lactamase-producing Pseudomonas aeruginosa (P. aeruginosa) is a major pathogen in burn wounds, often exhibiting high levels of antibiotic resistance, which complicates treatment strategies. This study deals with the validity of the modified Carbapenem Inactivation Method (mCIM) and the simplified Carbapenem Inactivation Method (sCIM) phenotypic tests for screening metallo-beta-lactamase (MBL) production by P. aeruginosa isolates from a referral burn center in Iran. Forty isolates were obtained between January and June 2021 and identified using conventional biochemical methods. Antimicrobial susceptibility testing was conducted following Clinical and Laboratory Standards Institute (CLSI) 2021 guidelines. mCIM based on CLSI 2023 guidelines was used to detect carbapenemase production. sCIM was also used based on previously developed protocols. PCR was performed to detect blaIMP, blaVIM, and blaNDM genes. The results were analyzed using SPSS and MedCalc. We observed a 90% resistance rate to imipenem and high resistance to other antibiotics, with multidrug-resistant (MDR) strains constituting 95% of the isolates. The mCIM test demonstrated high sensitivity (87.50%) and high negative predictive value (89.47%) and moderate specificity (70.83%) and moderate positive predictive value (66.67%) for detecting MBLs. In contrast, the sCIM test was unreliable, indicating a need for more standardized testing protocols. This study underscores the importance of accurate and timely detection of carbapenemase production to guide effective treatment.

Prevalence and clinical significance of the genotypic carriage among ESBL phenotype-negative Escherichia coli and Klebsiella pneumoniae clinical isolates in bacteremia: a study in a Malaysian tertiary center

Background

Antimicrobial resistance (AMR) can lead to fatal consequences. AMR genes carriage by phenotypically susceptible bacteria, such as Extended-Spectrum β-Lactamases (ESBL)s in Enterobacteriaceae, have potential implications for AMR spread and therapeutic outcomes. This phenomenon should be investigated.

Methods

Positive blood cultures from hospitalized patients in a Malaysian tertiary center between April 2022 and March 2023 were reviewed. A total of 137 clinical isolates of Escherichia coli (E.coli), Klebsiella pneumoniae (K.pneumoniae), and Klebsiella oxytoca were included. The antibiotic susceptibility and ESBL phenotypes were determined by disk diffusion method and the identification of genotypes by multiplex polymerase chain reaction. The clinical characteristics and outcome information were extracted by reviewing patients' medical records to evaluate the clinical significance of the ESBL genotype-positive but phenotype-negative isolates in bacteremia.

Results

All 137 isolates were positive for at least one genotype (bla CTX-M, n = 71, 51.8%; bla SHV, n = 87, 63.5%; bla TEM, n = 95, 69.3%; bla OXA-1, n = 38, 27.7%). While bla CTX-M was proportionately higher in the ESBL phenotype-positive isolates than ESBL phenotype-negative isolates (33/37, 89.2% vs 38/100, 38%; p < 0.001), more than half of those harboring bla CTX-M remained susceptible to third-generation cephalosporins (3GC). The sensitivity (Sen) of bla CTX-M for ESBL phenotypes prediction was 89.19% (95% confidence interval [CI], 74.58 - 96.97%); however, specificity (Sp) was low (46.47%; 95% CI 39.75 - 53.32). The patient characteristics were similar among 98 ESBL phenotype-negative cases, except that the non-bla CTX-M carrier group had significantly more renal impairment (0/37 vs 7/61, p = 0.043) and gastrointestinal sources of bacteremia (9/37 vs 27/61, p = 0.047). No differences were observed in infection severity, in-hospital mortality, and length of stay (LOS) between the bla CTX-M and non-bla CTX-M carrier groups.

Conclusion

The current study provides insight into the gene carriage in E.coli and Klebsiella species clinical isolates, including bla CTX-M genotypes in antibiotic-susceptible strains from a Malaysian hospital. The ESBL encoding genotypes such as bla CTX-M presented substantially beyond one-third of the ESBL phenotype-negative or 3GC susceptible E.coli and K.pneumoniae isolated from bloodstream infection. Although clinical outcomes were not worsened with bla CTX-M genotype-positive but ESBL phenotype-negative isolates in bacteremia, the potential implications for AMR spread deserve further investigation.

Clinically Isolated β-Lactam-Resistant Gram-Negative Bacilli in a Philippine Tertiary Care Hospital Harbor Multi-Class β-Lactamase Genes

In the Philippines, data are scarce on the co-occurrence of multiple β-lactamases (BLs) in clinically isolated Gram-negative bacilli. To investigate this phenomenon, we characterized BLs from various β-lactam-resistant Klebsiella pneumoniae, Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa isolated from a Philippine tertiary care hospital. The selected Gram-negative bacilli (n = 29) were resistant to either third-generation cephalosporins (resistance category 1 (RC1)), cephalosporins and penicillin-β-lactamase inhibitors (RC2), or carbapenems (RC3). Isolates resistant to other classes of antibiotics but susceptible to early-generation β-lactams were also selected (RC4). All isolates underwent antibiotic susceptibility testing, disk-diffusion-based BL detection assays, and PCR with sequence analysis of extended-spectrum BLs (ESBLs), metallo-BLs, AmpC BLs, and oxacillinases. Among the study isolates, 26/29 harbored multi-class BLs. All RC1 isolates produced ESBLs, with blaCTX-M as the dominant (19/29) gene. RC2 isolates produced ESBLs, four of which harbored blaTEM plus blaOXA-1 or other ESBL genes. RC3 isolates carried blaNDM and blaIMP, particularly in three of the metallo-BL producers. RC4 Enterobacteriaceae carried blaCTX-M, blaTEM, and blaOXA-24-like, while A. baumannii and P. aeruginosa in this category carried either blaIMP or blaOXA-24. Genotypic profiling, in complement with phenotypic characterization, revealed multi-class BLs and cryptic metallo-BLs among β-lactam-resistant Gram-negative bacilli.

Carbapenemase-producing Pseudomonas aeruginosa -an emerging challenge

Carbapenem-resistant Pseudomonas aeruginosa (CR-PA) is a major healthcare-associated pathogen worldwide. In the United States, 10–30% of P. aeruginosa isolates are carbapenem-resistant, while globally the percentage varies considerably. A subset of carbapenem-resistant P. aeruginosa isolates harbour carbapenemases, although due in part to limited screening for these enzymes in clinical laboratories, the actual percentage is unknown. Carbapenemase-mediated carbapenem resistance in P. aeruginosa is a significant concern as it greatly limits the choice of anti-infective strategies, although detecting carbapenemase-producing P. aeruginosa in the clinical laboratory can be challenging. Such organisms also have been associated with nosocomial spread requiring infection prevention interventions. The carbapenemases present in P. aeruginosa vary widely by region but include the Class A beta-lactamases, KPC and GES; metallo-beta-lactamases IMP, NDM, SPM, and VIM; and the Class D, OXA-48 enzymes. Rapid confirmation and differentiation among the various classes of carbapenemases is key to the initiation of early effective therapy. This may be accomplished using either molecular genotypic methods or phenotypic methods, although both have their limitations. Prompt evidence that rules out carbapenemases guides clinicians to more optimal therapeutic selections based on local phenotypic profiling of non-carbapenemase-producing, carbapenem-resistant P. aeruginosa. This article will review the testing strategies available for optimizing therapy of P. aeruginosa infections.

Using Molecular Diagnostics to Develop Therapeutic Strategies for Carbapenem-Resistant Gram-Negative Infections

Infections caused by multidrug-resistant Gram-negative organisms have become a global threat. Such infections can be very difficult to treat, especially when they are caused by carbapenemase-producing organisms (CPO). Since infections caused by CPO tend to have worse outcomes than non-CPO infections, it is important to identify the type of carbapenemase present in the isolate or at least the Ambler Class (i.e., A, B, or D), to optimize therapy. Many of the newer beta-lactam/beta-lactamase inhibitor combinations are not active against organisms carrying Class B metallo-enzymes, so differentiating organisms with Class A or D carbapenemases from those with Class B enzymes rapidly is critical. Using molecular tests to detect and differentiate carbapenem-resistance genes (CRG) in bacterial isolates provides fast and actionable results, but utilization of these tests globally appears to be low. Detecting CRG directly in positive blood culture bottles or in syndromic panels coupled with bacterial identification are helpful when results are positive, however, even negative results can provide guidance for anti-infective therapy for key organism-drug combinations when linked to local epidemiology. This perspective will focus on the reluctance of laboratories to use molecular tests as aids to developing therapeutic strategies for infections caused by carbapenem-resistant organisms and how to overcome that reluctance.