Automated Culturing Systems

Blood culture has evolved over the years from manual methods, now infrequently used, to automated culturing systems. The major advantage of an automated system, such as BACTEC NR660, is the obviation of manual inspection or examination to detect microbial growth because each system automatically does so by monitoring microbial CO2 production. Agitation of culture bottles also improves mixing and aeration to promote the growth of aerobes and facultative anaerobes. These features make blind subcultures of negative bottles unnecessary, as shown in a few studies reviewed by Reimer et al. (1997). Automation has improved the practice of blood culture enormously.

Continuously monitoring blood culturing systems (CMBCSs) are most commonly used nowadays. Introduced in the early 1990s, CMBCSs have added nearly continuous (every 10 to 12 min) monitoring of bacterial growth to the automated systems. Currently, three CMBCSs are available in the United States, and they are briefly shown in Table 1.1. More detailed description can be found elsewhere (Weinstein and Reller, 2002).

Numerous studies have been published to compare the performance of the systems and associated media with or without various lytic agents or additives to remove blood antimicrobials, and several recent ones are summarized as follows (Table 1.2).

Table 1.1. Commercial continuously monitoring blood culturing systems (CMBCSs).

Current system since

Microbial detection

Test interval

Newer

Manufacturer

early 1990s

mechanism

(min)

system, year

BioMerieux

BacT/Alert series for

Colorimetric for CO2

10

BacT/Alert

varying holding capacity

production

3D, 2001

Becton-Dickinson

BACTEC series for

Fluorescent for CO2

10

BACTEC LX,

varying holding capacity

production

2004

Trek

ESP series for varying

Manometric for CO2

12

VersaTrek,

holding capacity

production

2004

McDonald et al. (1996) compared the BacT/Alert standard bottle with BacT/ Alert FAN bottle that contains Ecosorb, an antimicrobial-absorbing substance, and they found that FAN bottle recovers significantly more microbes from all septic episodes, especially S. aureus, CoNS, and members of Enterobacteriaceae. Along with this, however, recovery of all contaminants, including CoNS, is also higher. The performance of the BacT/Alert FAN bottle and the BACTEC Plus aerobic/F bottle (with resins to absorb antimicrobials) were also compared, and the two systems were found comparable (Jorgensen et al., 1997). A recent study compared BacT/Alert standard bottle and BACTEC standard bottle and found the former significantly improved the recovery of S. aureus, CoNS, and yeasts (Mirrett et al., 2003). In a study comparing BacT/Alert FAN versus Trek ESP 80A, Doern et al. (1998) found that BacT/Alert FAN recovered more S. aureus, enterics, and non-Pseudomonas aeruginosa Gram-negative rods, along with more

Table 1.2. Performance of culture media with or without lytic agents or additives.

Compared media (bottle)

Findings

Reference

BacT/Alert FAN vs.

Comparable

Jorgensen et al., 1997

BACTEC Plus/F

BacT/Alert FAN vs.

BacT/Alert FAN improved recovery of

McDonald et al., 1996

BacT/Alert standard

S. aureus, CoNS, and enterics

BacT/Alert standard vs.

BacT/Alert standard improved recovery

Mirrett et al., 2003

BACTEC 9240 standard

of S. aureus, CoNS, and yeasts

BacT/Alert FAN vs. Trek

BacT/Alert FAN improved recovery of

Doern et al., 1998

ESP 80A

S. aureus, enterics, and

non-Pseudomonas aeruginosa

Gram-negative rods

BacT/Alert FAN vs. Trek

Overall comparable. BacT/Alert FAN

Welby-Sellenriek et al.,

ESP 80A, in pediatric

better for S. aureus and

1997

patients

antibiotic-treated samples; ESP 80A

better for streptococci and enterococci.

BacT/Alert FAN vs.

Comparable with detect fungemia.

McDonald et al., 2001

BACTEC fungal medium

BACTEC Plus Anaerobic/F

BACTEC Plus Anaerobic/F bottles

Wilson et al., 2001

bottles vs. Standard

detected more microorganisms

Anaerobic/F bottles

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 CO2 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.

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