J Clin Pathol: first published as 10.1136/jcp.38.8.926 on 1 August 1985. Downloaded from J Clin Pathol 1985;38:926-928 Evaluation of Malthus 128H microbiological growth analyser for detecting significant bacteriuria TK SMITH, R EGGINGTON, AA PEASE, DM HARRIS, RC SPENCER From the Department ofBacteriology, Royal Hallamshire Hospital, Sheffield SUMMARY In a pilot study a prototype Malthus 128H microbiological growth analyser was used to study the growth of six commonly encountered urinary pathogens in a range of five different laboratory media. No single medium had been able to show acceptable conductance changes for all bacteria within five hours. Subsequently, the Malthus system, with fastidious anaerobe broth as the culture medium, was used to screen 500 consecutive clinical specimens of urine for significant bacteriuria. After two and a half hours the Mathus system detected 32 out of the 40 true positives and eight of the 44 contaminated urines. Increasing the detection time to five hours permitted the recognition of 39 true positives and 37 contaminated urines. Conventional methods for diagnosing urinary tract teriuria.8 We report the results of a further investiga- infection entail the quantitative or semi quantitative tion in which we evaluated the Malthus system in culture of specimens of urine on solid culture media. 500 urine specimens in comparison with our stan- copyright. Results are read after overnight incubation, and a dard laboratory method. pure growth of a single bacterial species in numbers > 105 organisms/ml is accepted as indicative of infec- Material and methods tion.' Diagnostic laboratories investigate thousands of urine specimens each year but of these only a MALTHUS SYSTEM small proportion (about 10%) prove to be infected. The machine used was a prototype Malthus 128H Large amounts of both staff time and culture microbiological growth analyser9 controlled by a http://jcp.bmj.com/ medium are consequently expended on specimens Motorola exorset microcomputer with peripheral that eventually prove to be sterile. units of a matrix printer (ET-5200) and a digital Many screening methods have been advocated for plotter (Hi-Plot). The thermostatically controlled use in detecting bacteria in urine. These include water bath contained 128 growth cells. The compu- measurements of bacterial adenosine triphosphate ter was programmed for a total scan time of five by luciferase,2 measurement of heat generated by hours and each of the cells was scanned with a 10k metabolising organisms,3 changes in electrical impe- Hz signal every six minutes. Data from each cell on September 29, 2021 by guest. Protected dance,4 and three automated photometric were available on demand either on a visual display systems-Autobac (General Diagnostics), Auto unit or on the digital plotter as a graph which indi- Microbic System (Vitek Systems Inc), and MS-2 cated the change of conductance over time. Prelimi- (Abbott Laboratories)-which detect growth by nary work had shown that a suitable detection changes in light transmission5 and also particle routine entailed three successive increases of con- counting.6 The Malthus system measures the con- ductance of 4 uS. ductance between two electrodes immersed in media. Growth of bacteria in the culture is indicated MEDIA by a change in the electrical impedance of the Five media, Mueller-Hinton, brain-heart infusion, growth medium that has been shown to be propor- Todd-Hewitt, Isosensitest (Oxoid), and fastidious tional to changes in the conductance component of anaerobe broth (FAB, Lab M) were tested for their the impedance.' Brown and Warner examined the ability to support detectable growth by the Malthus possibility of using the Malthus system to detect bac- system using one strain each of the following com- monly encountered urinary pathogens: Escherichia coli, Pseudomonas aeruginosa, Streptococcus Accepted for publication 8 May 1985 faecalis, group B streptococcus, Staphylococcus 926 J Clin Pathol: first published as 10.1136/jcp.38.8.926 on 1 August 1985. Downloaded from Evaluation of Malthus 128H microbiological growth analyser for detecting significant bacteriuria 927 aureus, and Staphylococcus saprophyticus. All bac- After investigation into the most suitable media terial strains had been taken from clinical specimens fastidious anaerobe broth (without urea) was the of infected urine. As the accepted criteria for uri- medium chosen to screen 500 consecutive clinical nary tract infection is >105 organisms/ml suspen- specimens of urine. Of the 500 urines, 40 yielded a sions containing this amount were used to inoculate significant (>105 organisms/ml) pure growth of bac- the system, 002 ml being added to each cell. In teria by the laboratory's standard method: 37 preliminary experiments, 10 ml growth cells had Enterobacteriaceae; one Ps aeruginosa; one S been used. Using these, it was found that only E coli faecalis; and one Staph saprophyticus. A further 44 gave an adequate response. A change to the alterna- specimens yielded a growth of mixed bacteria at tive, smaller 2 ml growth cells produced a pro- levels of > 105 organisms/ml. The table shows the nounced improvement in time taken for detection of cumulative totals. At two and a half hours 80% of all the test organisms. the samples with significant bacteriuria were detected but so were 18% of the contaminated URINE SAMPLES urines. By extending the detection time the number Five hundred consecutive urine specimens were cul- of contaminated urines detected increased greatly. tured on CLED agar using a standard 0.01 ml Of the eight infections not detected by the Malthus nichrome wire loop.'0 A pure growth in excess of 105 system within two and a half hours, five were due to organisms/ml was regarded as confirmation of infec- E coli (the bacterial growth having been inhibited by tion and all organisms were identified by standard the presence of boric acid in the container), one Ps techniques." 12 The growth cells of the Malthus sys- aeruginosa, one S faecalis, and one Staph sap- tem were inoculated by "dipping" the ceramic strip rophyticus. containing the platinum electrodes into the speci- men. The ceramic strip was then replaced into the Discussion cell and connected to the apparatus by means of push on connectors. Previous work had determined We evaluated the Malthus system for its ability to that 0-02 ml of sample was consistently transferred detect the growth of potential urinary pathogens in copyright. from specimen to cell using this "dip" technique. liquid media, and subsequently, in the examination Each specimen was incubated for up to five hours, of 500 samples of urine. No single medium was suit- with every cell being subcultured on to a Columbia able for the detection of all six test organisms. If the blood and CLED agar plate at the end of the incuba- variables for conductance change were adjusted to tion. allow for greater sensitivity the growth of all test organisms was detected but the discrimination was Results reduced to such a degree that even uninoculated http://jcp.bmj.com/ broth gave false positive results. Fastidious Within five hours brain-heart infusion, Mueller- anaerobe broth seemed to be the best medium but Hinton, and Isosensitest media were unable to sus- performed badly in the detection of Staph sap- tain detectable growth of Ps aeruginosa and Staph rophyticus. This problem was alleviated by the addi- aureus. Todd-Hewitt and fastidious anaerobe broth tion of 1% urea, but this retarded the detection of required the shortest time to detect the six test Ps aeruginosa. The addition of 1 % urea was stopped organisms. Using these two media, E coli, S faecalis, because of this effect and also because, as it is heat on September 29, 2021 by guest. Protected and group B streptococcus were detected within one labile, urea has to be added after the autoclaving of and a half hours; Staph aureus, Staph saprophyticus, the growth cells, which increases the risk of con- and Ps aeruginosa were detected in four hours. tamination. Staph saprophyticus could be detected within one The original 10 ml cells were replaced by the and a half hours when urea was added to the smaller 2 ml growth cells, which enhanced the medium at a final concentration of 1%. The addition growth of all organisms. Enhanced growth could of urea, however, prolonged the time needed to have been due to either a higher initial concentra- detect Ps aeruginosa. tion of organisms or to an increased head space. For Time required to detect organisms using Malthus system and conventional urine cultures Conventional culture Detection (hours) (>1 050rganisms/ml) 05 1 15 2 25 3 3.5 4 4-5 5 Pure growth (n = 40) 8 15 25 27 32 35 36 37 39 39 Mixed growth (n = 44) 1 1 3 8 8 14 16 22 35 37 J Clin Pathol: first published as 10.1136/jcp.38.8.926 on 1 August 1985. Downloaded from 928 Smith, Eggington, Pease, Harris, Spencer routine clinical use of the system, it would be neces- We thank Malthus Instruments Ltd for technical sary to determine a cut off time that would distingu- cooperation and loan of equipment. ish between true positives-that is, patients with significant bacteriuria-and false positives--that is, contaminated specimens. If this cut off time were set at two and a half References hours, 80% of the true positives would be detected Kass EH. Bacteriuria and the diagnosis of infection of the urinary but so would 18% of the contaminated urines. To tract. Arch Intern Med 1957; 100:709-14. detect 100% of the true positives the cut off time 2 Mackett D, Kessock-Philip S, Bascomb S, Easmon CSF. Evalua- would have to be increased to five hours as has tion of the Lumac kit for the detection of bacteriuria by been bioluminescence.