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|Year : 2015 | Volume
| Issue : 5 | Page : 59--60
The last mile problem
Department of Microbiology, Himalayan Institute of Medical Sciences, Dehradun, Uttarakhand, India
Department of Microbiology, Himalayan Institute of Medical Sciences, Dehradun, Uttarakhand
|How to cite this article:|
Chatterjee B. The last mile problem.Indian J Med Microbiol 2015;33:59-60
|How to cite this URL:|
Chatterjee B. The last mile problem. Indian J Med Microbiol [serial online] 2015 [cited 2020 Jan 20 ];33:59-60
Available from: http://www.ijmm.org/text.asp?2015/33/5/59/150886
When I went home last month, the plane flew the 1300-odd kilometers from Delhi to Kolkata in less than 2 hours but then it took me another 2 hours to drive home, a mere 30 kilometers from the airport.
The study by Bhatacharya in the current issue  reminded me of a similar paradoxical situation that has arisen with the advent of automated blood culture systems such as BacT/Alert (bioMérieux), BACTEC (Becton Dickinson) or ESP (TREK Diagnostic Systems) in clinical microbiology.  These systems have dramatically cut down the time needed to detect growth, so much so that slow-growing and nutritionally-exacting bacteria such as Brucella spp. often grow out by the third or fourth day after inoculation, something that was unimaginable with older techniques.
However, our ability to rapidly detect growth in blood culture bottles has not had a commensurate impact on the turn-around time for blood cultures as a whole. Although modern technology allows us to detect growth in the least possible time, we fritter away that advantage by subsequently using identification and antimicrobial-resistance testing techniques that are decades older. In other words, we fly to our destination airport in a jiffy, and then spend as much time waiting for the suitcases to show up at the baggage carrousel. A long wait at the airport is tedious, but a long wait for a blood culture report can make all the difference between life and death. 
Needless to say, attempts have been made over the years to solve this problem in a variety of ways.
One such technique is differential centrifugation, wherein 10-mL of the culture broth is first centrifuged at low speed, e.g. 150 g, for 10 minutes to sediment blood cells.  The supernatant is then removed and centrifuged again, this time at 1000 g for 15 minutes to pellet the bacteria. The pelleted bacteria are resuspended in 0.5 mL of sterile normal saline and used as inoculum for identification and drug-resistance tests.
Kinetically limited differential centrifugation is an experimental technique, in which a sample of the positive blood culture broth is loaded on a Histopaque 1083 column and centrifuged.  The RBC and the bacteria, both of which are heavier than the broth and Histopaque, migrate through the Histopaque column. Bacteria are separated from the RBC by stopping centrifugation immediately after all the RBC have pelleted, at a time when the bacteria are still suspended in Histopaque. Although conceptually elegant, this technique is difficult to implement and is unlikely to become popular in its present form.
The Isolator lysis centrifugation system (Wampole Laboratories) is a manual blood culture method that bypasses the problem of purifying bacteria or fungi from broth altogether. A specimen of blood in EDTA is treated with saponin to lyze blood corpuscles but leaves bacteria and fungi unharmed. The lysate is then centrifuged to pellet the microbes, which are inoculated on agar media and incubated until colonies appear. The system performs extremely well for mycobacteria and fungi, but at the cost of significant hands-on time and a higher than usual risk of specimen contamination with environmental microbes during the lengthy processing time.
It is obvious that the solutions discussed so far are either expensive or time consuming, so attempts have been made to use the positive blood culture broth directly as an inoculum for drug-resistance testing. This approach saves an entire day, although results still need to be confirmed by conventional methods, firstly because not all positive blood cultures are monomicrobial, and secondly because of the difficulty in standardising the inoculum from blood culture broths.
In this context, an article in the current issue of IJMM, is highly relevant. The article, coming from a tertiary-level cancer centre, compares standard techniques for antimicrobial-resistance testing against a method endorsed by the British Society for Antimicrobial Chemotherapy (BSAC) that uses positive blood culture broths as the inoculum for drug-resistance tests and reports categorical and essential agreement in 84.88 and 97.38% of cases, respectively, among the 840 antibiotic-Gram negative organism combinations that were tested.
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