J Clin Pathol: first published as 10.1136/jcp.36.7.823 on 1 July 1983. Downloaded from J Clin Pathol 1983;36:823-828 Early detection of bacterial growth in blood culture by impedance monitoring with a Bactometer model 32 ANNE BUCKLAND, STUART KESSOCK-PHILIP, SHOSHANA BASCOMB From the Department ofMedical Microbiology, St Mary's Hospital Medical School, London, W2 1PG SUMMARY A Bactometer model 32 was evaluated for use in early detection of bacterial growth. Experiments with simulated cultures showed that 2 ml of broth introduced into the Bactometer module wells could detect 102 and 106 CFU/ml in 6 h and 2 h respectively. Both Brain Heart Infusion (BHI) and Fastidious Anaerobic broths supported good growth. Detection of nine of 10 organisms inoculated at approximately 106 CFU/ml in BHI were detected within 8 5 h. A culture of Bacteroides fragilis failed to grow under these conditions. Of 189 blood cultures, tested by incubation of 2 ml of BHI, 18 were positive by both conventional and Bactometer methods. False-positive or false-negative specimens were not observed using the Bactometer. Use of the Bactometer enables growth detection at least 12 h earlier than culture methods. Rapid detection of septicaemia is of primary impor- Material and methods tance in the diagnostic laboratory. Immediate Gram copyright. staining of blood specimens can detect approxi- CULTURE MEDIA mately 105 organisms/ml. However, culture methods Two blood culture media were used. Brain Heart are necessary to detect pathogens present at lower Infusion broth (BHI) + thymidine + sodium concentrations. Conventional methods rely on cul- polyanethyl sulphonate (SPS) and Fastidious turing of blood specimens and detection of growth Anaerobic broth (FAB) + thymidine, both supplied by subculturing and Gram staining requiring 18-24 by Lab M, Salford, Lancs. h for the first observation, negative cultures being http://jcp.bmj.com/ kept for seven days or more. More sensitive TEST CULTURES methods are therefore sought for faster detection of All strains were recent laboratory isolates identified positive blood cultures. Newer detection methods by starrdard technique.'3 available were reviewed by Bascomb.' Methods applied for detection of septicaemia include BLOOD CULTURES radiometric techniques2 measurement of bacterial Five ml of patient's blood was inoculated into 75 ml ATP3 and monitoring of impedance changes.4-6 A of BHI and FAB bottles on the ward. The blood on September 29, 2021 by guest. Protected number of impedance measuring devices have been culture set was brought to the laboratory immedi- described.' The Bactometer7 is commercially avail- ately and incubated at 37°C. /8-lactamase (0.5 ml) able and has been used for detection of bacteriuria8 9 (Genzyme Biochemicals) was added to each bottle if and in the food industry'0 and sewage effluent the patient was receiving a /8-lactam antibiotic. All monitoring."I Its use in detection of septicaemia has blood cultures were subcultured onto blood agar been limited.'24-6 We have evaluated the Bacto- anaerobically and chocolate agar in 7% CO2 after meter model 32 as to its suitability for monitoring 24 h, 48 h and seven days incubation at 37°C. Any growth of bacteria in blood cultures from the turbid bottle after 24 h incubation had a Gram film diagnostic laboratory. We also investigated the made and examined. effects of type and volume of medium, concentration of inoculum, and the applicability of VIABLE COUNTS the instrument to the variety of organisms likely to Miles and Misra counts'4 were performed on the be found in blood cultures. experimental broth cultures. IMPEDANCE MEASUREMENTS Accepted for publication 26 January 1983 The Bactometer consists of a thermostatically con- 823 J Clin Pathol: first published as 10.1136/jcp.36.7.823 on 1 July 1983. Downloaded from 824 Buckland, Kessock-Philip, Bascomb trolled monitoring unit. Four plastic modules (100 repeatedly traverse the channel width in either x 75 mm) each having eight pairs of rectangular direction, they then flatten out but still tend to drift wells can be accommodated in the incubator. Each slightly. This response, seen in all the blood cultures well contains a pair of electrodes that are connected examined, is thought not to be due to growth but to the monitoring unit via metal strips on the under- rather to the erythrocytes settling to the bottom of side of the module. The metal strips converge in a the well. tab that is plugged into the incubator. The periods in which growth responses were The modules come in a sterile plastic envelope, observed varied from 5 to 18 h, and in two cultures, the wells of which are covered by a plastic film, Clostridum perfringens and Staphylococcus epider- through which broth can be introduced aseptically midis two growth periods were observed, with a with a syringe and needle. After filling, another period of several hours of no growth in between. sterile adhesive film supplied with each module, is All but one of the organisms selected showed placed over the wells to seal the needle holes. growth within 8-5 h. The times for appearance of Each well can hold 2'5 ml of broth but, to avoid detectable growth are shown in Table 1. The failure overfilling, 2-0 ml was used. The lower row of wells, to detect growth of Bacteroides is probably due to filled with sterile broth, were used as reference, the the poor anaerobic conditions in the small volume of upper row of wells were filled with inoculated broth broth. or media taken from inoculated blood cultures. All eight wells of the module need not be filled EFFECT OF INOCULUM SIZE ON SPEED OF before monitoring commences, further cultures can DETECTION be added as and when they arrive in the laboratory. Dilutions of Escherichia coli, Haemophilus influen- Monitoring of impedance can also be achieved by zae and Staphylococcus aureus were prepared in inserting Bactometer electrodes into conventional BHI to give initial concentrations of approximately growth vessels. These are reusable autoclavable 1 x 102, 1 x 104 and 1 x 106 CFU/ml. Viable electrodes that are incorporated into a screw cap counts"4 were performed on these broths to establish that can replace the cap of the blood culture bottle. the exact concentrations. All wells were monitored copyright. This enables the bottles to be connected to an exten- for 6 h. As can be seen from Fig. 2 there is a good sion socket inside an ordinary incubator which correlation between inoculum size and detection relays information back to the Bactometer. Each time; 107 CFU/ml were detected in 1.5 h, 102 CFU/ blood culture, bottle requires a reference bottle of ml in 6 h. sterile broth with inserted electrodes. The electrodes can be reused about 30 times. EFFECT OF CULTURE MEDIA ON SPEED OF The impedance changes are shown as a trace on a DETECTION http://jcp.bmj.com/ chart recorder. Each trace is a representation of a Eight blood cultures were chosen to show differ- growth curve. To fit 32 curves onto one width of a ences between the FAB and BHI media. They were chart recorder paper, each curve is divided into monitored for 48 h and their traces examined. Six segments 8 mm wide (Fig. la). The rate at which the blood cultures showed no response, two showed a trace traverses a channel width gives an indication of growth response. The duration and the shape of the the rate of growth (Fig. ib). growth curves were the same for both media. Bacterial growth is represented by the trace on September 29, 2021 by guest. Protected traversing a channel width from left to right whereas DETECTION OF GROWTH IN BLOOD CULTURE yeasts produce the opposite effect. SPECIMENS One hundred and eighty nine blood cultures were Results examined during a period of four weeks. A 2-0 ml aliquot was drawn from the BHI bottle with a RANGE OF ORGANISMS DETECTABLE BY THE. syringe and needle for examination on the Bactome- BACTOMETER ter as described above. These cultures were moni- Cultures of 10 organisms likely to be found in blood tored for 48 h before being discarded. Complete (Table 1) were inoculated into BHI to give an agreement was found between growth and impe- approximate concentration of 1 x 10 CFU/ml. Two dance methods. Eighteen specimens were found to ml of each broth were transferred to the wells in the be positive by both methods. The remaining 171 top row of the module and 2-0 ml of sterile BHI into were found to be negative by both. The list of the reference wells. The modules were monitored on species isolated is given in Table 3. The majority of the Bactometer for 48 h for signs of growth. cultures were detected within 3 h. One specimen As can be seen from Fig. ic there is an initial containing Streptococcus pyogenes was detected period, ranging from 0-5 to 3 h, in which the traces after 4-5 h, two specimens containing Staphylococ- J Clin Pathol: first published as 10.1136/jcp.36.7.823 on 1 July 1983. 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