Differentiating cultures between commensal and pathogenic - (Dec/27/2007 )

How are normal (commensal) or contaminant bacteria differentiated from those associated with true infection in lab samples / cultures?
for example if the lab were to receive a stool/blood/sputum sample how do they know which organisms are commensal and which ones are pathogenic?

sorry if this is the wrong place to ask this question.

Also do you know where I can look for further reading on this subject, where they have more info on clinical microbiology investigations

thank u in advance

the cdc is a good resource; NIH is a good resource - but for in-depth knowledge you may wish to consult the literature.

as a prior Senior Microbiologist at a public health lab, I would say that often the culprit in an infection is the one that outgrows everything else - but there are other, more scientific criteria. there is essentially an algorithm followed for each type of sample, in conjunction with knowledge about clinical presentation. for example, if you had sputum from an immunocompromised individual who was coughing up bright green phlegm, you would look for Pseudomal pneumonia infection before you would look for TB

also, once the initial isolation has been performed, biochemical tests will often allow you to differentiate between species. for example, coagulase test for Staph sp; coagulase+ S is often aureus and often pathogenic. molecular tests for virulence factors are also applicable, as well as serotyping for species and subspecies...there are many ways to do this.

thank you very much for your help,

i will check out your references, and what your own input has also been very helpful