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Streptococcus mutans, a major cause of dental caries and endocarditis, is easily dismissed by the inexperienced as a common non-pathogenic skin bacterial contaminant termed ‘diphtheroid’.
A man of 62 was admitted after three months of lethargy, malaise, night sweats and weight loss. Type 2 diabetes mellitus had been diagnosed at about the time these symptoms began and, on a home visit, his general practitioner had thought they were due to poor diabetic control. The patient had not experienced chest pain, cough, rigors or arthralgia. His medical history included a Ross procedure (pulmonary valve autograft to aortic position, pulmonary valve homograft) at age 35 for a congenital abnormality of the aortic valve. His medication was gliclazide and aspirin.
On examination he was pyrexial (38.28C), and tachycardic. The principal abnormal physical sign was a mid-systolic murmur. There was no rash or splenomegaly. Haemoglobin was 11.6 g/L, white cell count 21.66109/L (neutrophils 95%), C-reactive protein 72 mg/L, erythrocyte sedimentation rate 94 mm/L, random blood glucose 13.1 mmol/L, HbA1c 9.6%. The initial diagnosis was hyperosmolar non-ketotic hyperglycaemia. Because of his persistent pyrexia, a set of peripheral blood cultures was taken which became positive within 24 h showing Gram-positive rods. Since a biochemical test for the identification of Gram-positive rods gave no profile, the isolate was reported as ‘diphtheroids’ and presumed to be a contaminant. However, because of his previous valve replacement and his ongoing fevers the clinicians were advised to take more blood cultures. Three further sets of blood cultures became positive within two days. Gram-positive rods were again seen; and, on review of the original Gram films, the organisms were deemed more likely to be streptococci, possibly S. mutans. It was strongly suggested to exclude endocarditis in this patient and to review the state of his teeth. A transthoracic echocardiogram showed a small mobile vegetation on a thickened pulmonary valve. He was then treated for presumed streptococcal endocarditis with gentamicin 80 mg twice daily for two weeks and vancomycin twice daily for five weeks (he was allergic to penicillin). Eight teeth with advanced caries were removed while he was in hospital. His infection markers became normal.
Originally, the laboratory performed a biochemical test (API) for the identification of streptococci, which named the organisms as Leuconostoc sp. This was unlikely to be correct since the isolate was vancomycin sensitive whereas Leuconostoc spp. are resistant. After two weeks, the reference laboratory confirmed the isolate to be a fully sensitive S. mutans.
S. mutans belongs to a group of non-haemolytic Gram-positive cocci which typically appear rod-shaped on acid culture medium but show a streptococcal appearance, in chains, when subcultured into a neutral or alkaline broth. The organism was first described by Clarke in 1924, who noted the variation of morphology with the pH of the medium.1 On the basis of their rod-shaped appearance when isolated from blood culture medium they can be easily dismissed as diphtheroids—i.e. non-pathogenic skin bacteria. In 1977 Emmerson and Eykyn2 drew attention to this hazard, by reporting two cases of S. mutans endocarditis in which the organisms had been initially misidentified as diphtheroid contaminants.
Although diphtheroids are the second most common bacterial contaminants in blood cultures, the case presented here illustrates that 27 years on we still fall into the same trap of misidentifying an important pathogen and potentially delaying the diagnosis and treatment of a serious infection.3 Clinicians and microbiology staff need to be critical about the identification of ‘diphtheroids’ from blood cultures. New technology such as commercial identification kits may even add to the confusion.