Advanced Techniques in Diagnostic Microbiology

1. Automated Blood Cultures 7
contaminants, too. In a similar study in pediatric patients (Welby-Sellenriek et al.,
1997), the two systems were found to be overall comparable, with BacT/Alert
FAN recovering more S. aureus and better for antibiotic-containing samples and
ESP 80A recovering more streptococci and enterococci. Because yeasts are an
increasing cause of nosocomial bloodstream infections, McDonald et al. (2001)
compared BacT/Alert FAN with BACTEC fungal medium for their recovery, and
the two systems were found comparable. The anaerobic culture media have also
been compared; a recent study by Wilson et al. (2001) found that the BACTEC
Plus Anaerobic/F bottles detect more microorganisms and episodes of bacteremia
and fungemia than the BACTEC Standard Anaerobic/F bottles.
In summary, CMBCSs, each with cost, strength, and weakness, perform well
overall in delivering timely and accurate diagnosis of bloodstream infections. Addition
of lytic or antimicrobial-absorbing substances has consistently improved
the recovery of S. aureus and members of Enterobacteriaceae, particularly from
treated patients.
New versions of CMBCSs have been released or are about to be (Table 1.1),
which have kept the key elements from earlier versions while refining the hardware,
computer system, and data management. The trend is to increase user-friendly
features for space, operation, and flexibility. The BACTEC LX system now also
uses laser technology to detect microbial CO 2 production. Clinical evaluations
are being conducted and results are expected soon. It is reasonable to assume that
newer systems should perform just as well as or better than their previous versions.
Blood Culture for Mycobacteria
Bacteremia due to rapidly growing mycobacteria (RGM) can be detected by blood
cultures, similar to other fastidious organisms. In our experience with the BACTEC
9240 and the Isolator 10 system (Wampole Laboratories, Princeton, NJ, USA),
RGM typically grow in 2–5 days (De et al., 2005). RGM bacteremias are usually
associated with use of intravascular catheter (Raad et al., 1991; De et al., 2005).
From an analysis of 80 consecutive clinical RGM strains, 24 were isolated from
blood and/or catheters, and Mycobacterium mucogenicum accounted for most of
them (15 of 24) (De et al., 2005).
Blood culture has been useful to detect and monitor Mycobacterium avium bacteremia
in patients with AIDS. M. avium bacteremia usually occurs when the CD4 +
cell count falls below 50/mm 3 (Inderlied et al., 1993). Circulating M. avium, exclusively
within monocytes, usually range 10 to 10 3 colony-forming units (CFU)
per milliliter of blood but can be as high as 10 6 CFU/mL (Inderlied et al., 1993).
A number of blood culture systems have been used: the earlier BACTEC 13A
radiometric system and Isolator 10 system and the more recent CMBCSs, such as
BACTEC 9240 with MYCO/F LYTIC medium and BacT/Alert MB. Several studies
have evaluated these systems. In a controlled comparison of the performance of
these systems, Crump et al. (2003a) found that these systems perform comparably
well in detecting M. avium bacteremia and other mycobacterial and fungal sepsis.