Emerging hypervirulent and extensively drug-resistant Klebsiella pneumoniae with potent biofilm formation as a new clinical challenge.

Hypervirulent Klebsiella pneumoniae strains (hvKp) can induce invasive community-acquired infections in healthy individuals. A total of 71 K. pneumoniae isolates were obtained from patients admitted to teaching hospitals in Arak, Iran. The hypermucoviscous phenotype was assessed using the string test. The presumptive hvKp strains were identified by the iucA gene and screened for capsular genes (K1, K2, K5, K20, K54, K57) and virulence factors (rmpA, rmpA2, iroB, peg-344). Antimicrobial susceptibility and colistin broth disk elution were also conducted. The biofilm formation ability was evaluated using a microtiter plate assay. Of the isolates, 28.17% (20/71) were identified as presumptive hvKp. Among the 20 isolates, the K2 serotype was the most prevalent (35%), followed by K1 (25%) and K20 (20%). In addition, K54 and K57 serotypes were each identified in one isolate. The virulence gene rmpA2 was highly prevalent (80%), along with peg-344 (75%), iroB (70%), and rmpA (70%). Regarding antibiotic susceptibility, cefotaxime showed the weakest effect, with 100% resistance, followed by cefoxitin, ampicillin-sulbactam, meropenem, ceftazidime, and cefepime, each of which demonstrated a 90% resistance rate. Tetracycline and gentamicin were the most effective antibiotics, with susceptibility rates of 60% and 50%, respectively. Additionally, 30% (6/20) of isolates were confirmed as ESBL positive, and none exhibited resistance to colistin. Among isolates, 85% (17/20) were extensively drug-resistant (XDR). Moreover, 80% of presumptive hvKp isolates demonstrated biofilm formation capacity, with 20% showing strong biofilm formation. The emergence of biofilm-producing and XDR strains underscores the need for enhanced surveillance and effective infection control measures.