الفهرس 
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Abstract
Klebsiella pneumoniae is considered an important opportunistic multidrug-resistant pathogen. Infections caused by K. pneumoniae are such as pneumonia, urinary tract infection (UTI) and pyogenic liver abscess (PLA). The flexibility of this species has given it remarkable success in developing a multitude of medical infections that result in morbidity and mortality. This study was undertaken to study the prevalence of serotypes and compare the virulence characteristics and genetic relatedness between different clinical K. pneumoniae isolates. Seventy-three out of 191 of bacterial isolates collected from Mansoura hospitals were identified as K. pneumoniae. The resistance patterns of all the isolates were determined against 14 antimicrobial agents. K. pneumonia isolates exhibited high resistance to ampicillin, amoxicillin-clavulanic acid, piperacillin, amoxicillin, piperacillin/tazobactam, cefotaxime, ciprofloxacin and trimethoprim/sulfamethoxazole by the following percentages: 98.6%, 97.26 %, 97.26%, 93.15%, 57.53%, 94.52%, 57.53% and 71.23%, respectively. For gentamicin and streptomycin, the resistance was 50.68% for both of them while to tobramycin was 61.64%. On the other hand, K. pneumoniae isolates exhibited more susceptibility to amikacin and imipenem by percentage of 73.97% and 94.52%, respectively. Imipenem and amikacin were the most effective antibiotics against our isolates that could be effective in treatment. from resistance pattern, multidrug resistant isolates were scored 83.56% (61/73) of total isolates. Serological identification of capsular serotypes revealed that 93.1% of the tested isolates was serotyped. K1 was significantly the predominant serotype being shown by 79.45% while only 13.7% of isolates was K2 serotype. Phenotypic detection of different virulence factors was carried out. Some of virulence associated genes were genetically screened by PCR. The prevalence of virulence factors in K. pneumoniae isolates was found as the following: Serum resistance (95.9%), haemagglutination (63%), hypermucoviscosity (45.2%), biofilm formation (total 76.72%, strong 1.37%, moderate 16.43%, weak 58.9%), lipase enzyme production (19.2%), protease enzyme production (52%), lecithinase enzyme production (42.5%), wabG gene (94.5%), fimH gene (92%), rmpA gene (20.5%) and kfuBC gene (66%). Fifty-nine different virulence profiles were detected including 48 unique profiles and the others profiles were repeated in two to five isolates. The ability of K. pneumoniae to overcome the host immune defense leads to an effective phase of infection that avoids different pathways of immune response. Non-mucoid K. pneumoniae strains showed higher levels of complement deposition via the alternative pathway compared to hypermucoid strains. Hypermucoviscous strains resulted in a significantly higher bacterial load in lung tissues, which confirms the higher pathogenicity of this type of strains. This can explain how the mucoid strains evade complement mediated lysis and causing serious bacterial infection.