Viridans Group Streptococci
Viridans group Streptococci (VGS) are:
- alpha-haemolytic gram positive cocci belonging to the genus Streptococcus.
- part of the normal flora of the oral cavity, upper respiratory tract and gastro-intestinal system.
In the last 15 years, VGS has become a leading cause of bacteriaemia and sepsis in the immunocompromised host, in particular in children undergoing chemotherapy for AML and after haematopoietic stem cell transplantation (HSCT).
In 2 large multi-institutional studies of children with AML, VGS represented 22 to 25% of all bacteriaemic events 31,32. In a review of 36 cases of VGS bacteriaemia from 1991-2000, authors from St. Jude Hospital, Memphis, TN, noted a recrudescence of cases linked to a change of protocol using increased doses of cytosine arabinoside 33. High dose Ara-C has since been linked to the development of VGS sepsis in other studies 32,34. It is unclear if this is secondary to a direct effect of this chemotherapeutic agent or is the indirect result of the development of mucositis and the prolonged period of neutropaenia induced by this agent.
Patients with VGS bacteriaemia will often present with septic shock and acute respiratory distress syndrome (ARDS). The fever lasts on average 10-14 days, even though blood cultures become rapidly negative. Some authors have suggested a role for an inflammatory response triggered by the organism leading to some degree of cytokine release dysregulation 35. However, no toxin has ever been demonstrated to be present in VGS. These patients are also at increased risk to subsequently develop an invasive fungal infection (invasive pulmonary aspergillosis) during the same febrile episode33.
The increase in VGS septic episodes has led to an increase in the use of Vancomycin as part of the empiric regimen. VGS resistance to penicillin is variable reported from 4-14%, with intermediate susceptibility reported between 14-64% 36. Amongst β-lactam antibiotics, ceftazidime has been reported as having the least activity in vitro, with geometric mean MIC reported 15 folds higher than penicillin 37. It is therefore a reasonable consideration to start vancomycin empirically in AML patients who present with sepsis, before identification and susceptibilities are fully known.