Monthly Archives: September 2013

“Woolly Mammoths or Unknown Soldiers?”

The Science of Microbiology,

Woolly_mammoth

Do MRSA decolonisation regimes rid an MRSA carrier completely of the bacteria?

A person who is colonised with methicillin resistant Staphylococcus aureus bacteria (MRSA), at a rough guess, are likely to harbour between 10,000,000 and 10,000,000,000 MRSA bacteria in their bodies.

Should we believe then that a successful decolonisation regime rids the person completely of MRSA?, or just below the detection limits of the laboratory assay.

There are thus two possible theories:

Woolly mammoth: When unfavourable environmental conditions prevail, the decline in numbers in a population reaches a critical point where the decrease is irreversible and the numbers present in the population/host decline to zero.

Unknown soldier: An army regiment loses a battle. However the surviving soldiers hide away undectected in the population, ready to regroup and fight again when conditions are more favourable.

I have no idea what the answer is, but would be interested in any suggestions. My gut instinct is that MRSA decolonisation falls into the latter (soldier) theory, and that after “successful” decolonisation, a small number of MRSA bacteria lie undetected in the host (?intra-cellular) and undetectable, waiting to make a re-appearance when conditions become more favourable to their proliferation.

I am only making an educated guess here however….

 

Michael

“Spending money where it matters..”

Future of Microbiology, , ,

                                     Foreign Banknotes 2.jpg

I suspect the next revolution in diagnostic bacteriology will be (routine) rapid identification of bacteria from blood cultures which have flagged positive on blood culture analysers.

Fluorescent In-Situ Hybridsation (FISH) technology along with Maldi-TOF sepsityper, now allows us to identify the causative bacterium accurately in the majority of cases within an hour of the blood culture becoming positive. This has obvious positive clinical implications, particularly if the identified bacterium is a beta-haemolytic streptococcus, a Pseudomonas aeruginosa or a Staphylococcus aureus. Even rapidly identifying coagulase negative staphylococci may avert the need for unnecssary anti-microbial therapy.

These technologies are NOT cost-prohibitive, and should be within the scope and skill level of most reasonably sized diagnostic laboratories.

I believe it will be only a matter of time before it becomes unacceptable to wait for the bacterium to grow on agar plates before identifying it.

I think these technologies also have implications for how the microbiology lab operates overnight. If we can identify the organism within one hour then is it really acceptable to leave a positive blood culture unattended to in an analyser for 8-12 hours overnight? (This definitely still happens in many diagnostic laboratories..)

With these new technologies becoming commonplace, maybe it is time to re-think how we use our budgets and how we roster the microbiology staff. Maybe we need to divert the money used to work-up bacteria from peri-anal abscesses and chronic venous ulcers towards rapid identification of bacteria from positive blood cultures, where it is going to make a real clinical difference and be potentially life-saving.

…and maybe the blood culture analyser needs to be staffed 24 hrs a day, 7 days a week, with blood cultures being processed and bacteria identified as soon as possible.

These sort of changes might cause a few grumbles but this is the sort of direction we need to be heading in, in order to have a clinical microbiology service that makes a genuine difference to the patient…

Michael

p.s. In future, any links from the articles will be highlighted in purple, to avoid confusion

“Accepting Rejection”

The Science of Microbiology, , ,

Apologies for the recent sparcity of posts as I continue my futile efforts at finding a suitable apartment in Paris…

A recent paper in the Journal of Clinical Microbiology caught my interest regarding the use of rejection parameters for deciding whether or not to test CSF for Herpes Simplex Virus (HSV) by PCR, according to the white cell count and other clinical parameters.

Click here for the abstract. (Unfortunately the full article needs subscription to JCM)

More and more laboratories are implementing rejection criteria for testing HSV in CSF. The test is often requested before the cell counts and protein are known (ie on the initial request form), and often the CSF parameters are completely normal. 

The researchers put forward a case for only testing HSV PCR if CSF samples had >10 cells/mm3 or if the sample was from an immunocompromised patient or a child aged <2 years. 

The researcher’s paper looks convincing. However before we accept such criteria we need to reflect on how serious HSV infection of the CNS is, particularly HSV encephalitis. This is not a diagnosis where as a laboratory we can afford to miss a positive diagnosis. The consequences could potentially be catastrophic. See this article  for some sort of an explanation.

Very very occasionally, as previous similar papers have demonstrated (a pseudo meta-analysis if you like..), HSV encephalitis does occur where the child is older than 2 years, doe not have “classical” immunocompromise and CSF parameters are plum normal.

So while we strive for laboratory efficiency (which I strongly support), we need to be very careful in our interpretation of such papers, maintain close co-operation between the clinician and the laboratory, and be prepared to be flexible in our testing protocols for individual patients.

Michael