Characterisation of Coagulase-negative Staphylococcus Isolates Recovered from Blood Cultures of Patients in a Haematology and Bone Marrow Transplant Unit

Abstract

Infections are the most important cause of morbidity in patients with haematological malignancies and those undergoing haematopoietic stem cell transplantation. Coagulase negative Staphylococcus (CoNS) species are among the most frequently encountered isolates in this hospitalised bacteraemic patient group. The common therapeutic agent for the treatment of these clinically significant isolates is vancomycin. However, the emergence of resistance to other commonly used antibiotics and a shift towards the predominance to multi-resistance has resulted in new challenges to these opportunistic pathogens. Therefore, new therapeutic agents are required to combat the emergence of resistance for optimal clinical outcomes. This thesis is a study designed to characterise CoNS using phenotypic and genotypic methods together with determining the presence of important virulence factors, antibiotic-resistance including the magnitude of susceptibility to vancomycin and some of the newer class of antimicrobials used for the treatment of bacteraemic patients hospitalised for haematological malignancies. Characterisation and analysis was carried out on a total of 100 CoNS strains consecutively isolated from blood cultures of patients hospitalised in the haematology and Bone Marrow Transplant (BMT) unit of the Queen Elizabeth Hospital from April 2010 to March 2012. Comparative identification and susceptibility testing was also performed on 100 CoNS skin colonising strains obtained from a pre-admission patient (control) group. This study addresses the issue that skin colonisers could be regarded as the putative source of opportunistic bacteraemia in haematology patients. The key question being whether the CoNS strains as the aetiological agents of bacteraemia, originated from random carriage on the skin of the patients or were a result of distinct strains with enhanced virulence due to antibiotic pressure. Conventional and molecular techniques identified major phenotypic and genotypic differences with low-level similarities within and between in-patient blood culture isolates of CoNS and those skin-colonisers which could be regarded as inducers of bacteraemia. Biotyping revealed a total of eleven CoNS species of which Staphylococcus epidermidis was the most prevalent followed by Staphylococcus hominis and Staphylococcus haemolyticus in both patient groups. Likewise, slime the main virulent factor, was prevalent amongst both patient group S. epidermidis isolates. Interestingly, resistance to principal classes of commonly used anti-staphylococcal drugs was also species-specific with S. epidermidis demonstrating a propensity for both methicillin-resistance and multi-drug resistance. Significantly high levels of teicoplanin resistance among both S. epidermidis and S. haemolyticus in-patient isolates were also likely to express low-level or heteroresistance to vancomycin. Of the newer antibiotics evaluated, daptomycin demonstrated broad-spectrum in-vitro activity. The study focussing on clinically relevant combinations of vancomycin with rifampicin, gentamicin and daptomycin demonstrated either an additive or indifference effect with no antagonistic activity. Conversely vancomycin, daptomycin and rifampicin, individually and in combination demonstrated antagonistic interactions against S. epidermidis biofilm-embedded cells. Hence, these combinations may not be considered as potential adjunctive agents for the treatment of BSIs caused by biofilm-producing strains. Phenotypic and genotypic characterisation to ascertain the likelihood of skin-colonising strains as aetiological agents of bacteraemia revealed the presence of diverse heterogenic populations amongst all CoNS species-specific isolates of both patient groups. The findings of this study therefore do not support the hypothesis that endogenous skin-colonisers could be the potential source of CoNS-associated bacteraemia in the haematology patient group.