Blood cultures are one of the most significant investigations when they provide positive results. Bacteraemia is the presence of bacteria in the blood stream. The word “septicaemia” describes a clinical picture with shock associated with bacteria in the blood stream. The laboratory cannot strictly demonstrate septicaemia, only bacteraemia. The clinician responsible for the patient can however reasonably describe a patient with a bacteraemia as septicaemic because of the association of the laboratory finding and the clinical picture of a shock syndrome.
First Results on Microscopy from the Broth
With continuous monitoring incubators, blood culture positives can be identified in a very short time. As a general principle, the earlier a blood culture becomes positive, the greater the likely significance. Initially, the bacteria are growing in the broth of the blood culture bottle. We therefore have only an initial microscopy to guide interpretation. A Gram stain is performed and will give the initial guide:
Gram Positive Organisms
Gram positives are those that retain the stain and appear black. The shape of the bacteria is the next criteria. Bacterial spheres are described as the singular, “coccus” or plural, “cocci”. The cigar or rod shaped bacteria are described as the singular, “bacillus” or plural, “bacilli”. Finally, in terms of cocci, we need to know how they are arranged: either as “bunches or grapes” or “chains”. Gram positive cocci in bunches will normally be Staphylococci while Gram positive cocci in chains or pairs will normally be Streptococci.
Gram positive cocci, possible Staphs
Until the organism from the broth culture is growing on solid media, we get no more information and at this stage, the consultant microbiologist will seek to understand the result in the context of the patients clinical picture. The most significant Staph is Staph aureus, responsible primarily for skin infection but is also the classic abscess forming organism as well as a possible source for deep seated infection. The issue, of course is to consider whether the Staph isolated from the blood culture is significant or not and also to risk assess the likelihood of it being methicillin resistant or sensitive.
So often, the blood culture will have been taken in response to clinical infection. The commonest presenting pyrexial illnesses in hospital practice will be respiratory or urinary with skin and abdominal somewhere behind. Respiratory or urinary infection is not commonly due to Staph aureus but does occur. Thus, the initial assessment requires consideration of the nature of the infection and any antibiotics used to treat from the time of admission. For a respiratory infection, treatment will often include co-amoxiclav, which is as effective as flucloxacillin against methicillin sensitive Staphs. If the patient has responded to the initial antibiotics, then it is reasonable to continue until the more definitive results are available the following day. This is because the commonest contaminant in blood cultures are the coagulate-negative Staphs. These are normal skin flora and rarely of significance. Their principle significance is in association with medical devices. Thus, patients who have central intravenous lines, peripheral lines, implanted medical devices could become colonised with these skin type flora and potentially be reflected in the blood culture.
Broth from the positive bottle is spread on solid media for overnight culture, allowing the first opportunity to undertake further tests of identity the following day. Similarly, first attempts at sensitivity testing are set up again for overnight assessment.
Gram positive cocci, possible Streptococci
These Gram positive cocci will be arranged in chains or pairs. The Streptococci include those associated with skin and soft tissue infection, the beta haemolytic Streps of group A, C and G, those associated with pneumonia, the Strep pneumoniae (which from solid media will classically appear as diplococci i.e in pairs but which from a liquid are often in chains) and those associated with abdominal or urinary infections, the enterococci. Streptococci may also be associated with skin contaminants but microbiologists are a lot more cautious before assigning a strep as a contaminant. This is because the alpha haemolytic and non-haemolytic Streps from blood cultures may be the first indicator of endocarditis.
Again, when the first isolate from liquid culture is made, the microbiologist will seek to establish what the clinical findings in the patient suggest and any initial antibiotic therapy. The penicillins can usually be relied on for the treatment of streptococci but other antibiotics show variable sensitivity and may influence the advice the microbiologist will provide during this initial period.
Gram positive bacilli (or rods)
One of the significant organisms in this group that we would be thinking about would be possible Listeria, the cause of sepsis or meningitis in the neonatal period or meningitis in the elderly. If growing in the anaerobic bottle, this might be a Clostridium species and associated with something from an abdominal perspective or perhaps associated with gangrene. However, we also have Bacillus species in this category, which are most commonly environmental contaminants and Corynebacteria, which are skin flora. Both groups of course can be associated with colonisation of medical devices and the Bacillus species do include the very significant, if rare, Bacillus anthrasis, the cause of anthrax.
Gram negative bacilli
Gram negative bacilli are almost invariably significant. Organisms in the group include the coliforms, like E. coli which are common causes of urinary tract infection and abdominal associated infections. We may find them associated with cholecystitis or cholangitis, diverticultis, appendicitis etc. They can also be the presentation of the likes of pseudomonas etc. As with the Gram positive organisms, the initial contact with the ward or medical team associated with the patient will provide the first clues and whether we feel the antibiotics will offer cover for possible infections.
Gram negative cocci
The important isolate in this group is Neiserria meningitis, one of the principle causes or meningitis but also of meningococcal septicaemia.
Day two results when on solid media
The broth from the initial positive bottles is spread on a variety of agar plate media and incubated together with the initial sensitivity tests. At this stage, everything becomes a little clearer: the Staphylococci can be divided into Staph aureus (coagulase positive) and the coagulase negative group. The sensitivitiy tests will also indictate if a Staph aureus is meticillin sensitive or resistant. Similarly, streptococci can be divided into their various subtypes again with appropriate sensitivitties. For the Gram negative organisms we can determine the organism type and sensitivities.
If all these criteria fit clinically, a further report can be issued to indicate this and any follow up tests that might be required: further sensitivities, further identification, typing or other appropriate tests.