to know was what causes infections. This is a particularly common and understandable approach from students but misses the significance of the most fundamental principle of microbiology or in fact medicine. If you do not know what is normal, how do you recognise abnormal? I smile inwardly and count to ten before going on to explain the importance of this often overlooked bit of knowledge.
not indicate infection.
derived from a person’s normal flora getting in to a site it should not be in. These are called endogenous infections. This is either from direct extension from a nearby site or by distant spread. By knowing the normal flora for a site and what antimicrobials these micro-organisms are normally sensitive to, allows you to predict the cause of an infection and derive an appropriate treatment regimen. For example, pneumonia tends to be caused by bacteria from the URT;
knowing what the normal flora of the URT is allows prediction of the antibiotics necessary to treat pneumonia. What’s more by understanding how antibiotics will impact on this normal flora (e.g. remove sensitive bacteria leaving a void which could be filled by resistant bacteria); you can determine what the likely cause of an infection will be, in a patient who has recently received antibiotic treatment.
Secondly, knowing where micro-organisms normally live allows you to work out where a pathological process is taking place when you find that micro-organism in a normally sterile site. For example, the presence of E. coli
(which normally lives in the bowel) in a blood culture indicates a significant problem in the abdomen either in the bowel itself such as in colitis, or in an adjacent structure such as the urinary tract or biliary tree. Another example,
isolation of a Viridans Streptococcus in blood cultures may indicate infective endocarditis as a result of poor dentition as Viridans Streptococcus are part of the normal mouth flora.
It’s important that doctors adopt a questioning attitude and ask “why has my patient got this infection?” not just “what antibiotic am I going to treat my patient with?” Some people believe bacteria have no place in our environment but bacteria are part of the normal environment. Using the knowledge of where normal flora comes from gives you an advantage, allowing you to identify the likely cause of infection and help you to know where to investigate. This knowledge is known by microbiologists, that is why they seem to identify where the problem is without even seeing the patient!…Microbiologists are not better at discovering
infection just better at knowing where micro-organisms should and should not be and applying this knowledge.