J Clin Pathol: first published as 10.1136/jcp.20.6.870 on 1 November 1967. Downloaded from J. clin. Path. (1967). 870 Resistance to cloxacillin among hospital staphylococci. G. C. TURNER' AND P. E. COX From the Department ofPathology, Sefton General Hospital, Liverpool SYNOPSIS Cloxacillin-resistant strains of Staphylococcus aureus were detected by their ability to grow on agar containing 1 *6 ,ug./ml. ofcloxacillin, a more reliable method than the disc-diffusion test. At Sefton General Hospital, Liverpool, between 1963 and 1965, there was an increase in the number of infections among in-patients that were caused by staphylococci resistant to cloxacillin despite the fact that use of the antibiotic was largely confined to an isolation ward for patients with staphylococcal sepsis. Although there is no evidence yet that staphylococci resistant to cloxacillin will become as common in hospital practice as those resistant to penicillin and tetracycline it is clear that there is a need for continued vigilance and measures to prevent spread of staphylococci from infected patients. The last 20 years have seen the emergence of varieties additions to the list of antibiotics available for the of Staphylococcus aureus resistant to a succession of treatment of staphylococcal infections. Stewart and antibiotics as these have been introduced in turn in Holt (1963) reported at a children's hospital thecopyright. hospital practice. Since the appearance of the now isolation from 37 patients, most of them without commonplace penicillin-resistant staphylococci, active infections, of a staphylococcus of phage type which owe their resistance to the production of peni- 75 resistant to methicillin and cloxacillin. In a general cillinase, staphylococci have emerged which are hospital, over a 20-month period, Colley, McNicol, resistant to streptomycin, tetracycline, and usually and Bracken (1965) isolated from 73 patients 79 mercuric chloride; because such strains are now very strains of phage group III which were resistant to common among patients in hospital and much less methicillin and cloxacillin; 61 of these were of phage so among others, the term 'hospital staphylococci' is type 7/47/53/54/75/77. http://jcp.bmj.com/ often used to describe them. These strains are also Methicillin was first used at Sefton General Hos- often resistant to erythromycin and sometimes to pital in 1960; cloxacillin followed two years later and chloramphenicol; some are resistant to neomycin and because it was suitable for administration either by these almost always show unusual reactions with mouth or by injection, it replaced methicillin (Table typing bacteriophages (Jevons, John, and Parker, I). After 1962 it was the generally accepted policy in 1966). TABLE I These multiple-resistant strains usually produce on September 25, 2021 by guest. Protected large amounts of penicillinase (Richmond, Parker, METIICILLIN AND CLOXACILLIN DISPENSED AT SEFTON Jevons, and John, 1964) and special interest therefore GENERAL HOSPITAL attaches to the introduction and increasing use in Year Methicillin (g.) Cloxacillin (g.) Total (g.) hospitals of methicillin and cloxacillin (Celbenin) 1960 300 0 300 (Orbenin), the relatively penicillinase-resistant anti- 1961 4,050 0 4,050 biotics derived from the penicillin 'nucleus', 6-amino- 1962 3,700 386 4,086 1963 900 2,802 3,702 penicillanic acid. Because the extensive use of so 1964 100 4,750 4,850 many antibiotics for the treatment of infections in 1965 0 3,825 3,825 hospitals has been followed by the appearance of the hospital to use cloxacillin for the treatment of all large numbers of strains of Staph. aureus that are multiple-resistant staphylococcal In this resistant to them, it is clearly important to establish infections. with recent article we present a survey of infections caused by whether this can happen these valuable staphylococci resistant to cloxacillin which were en- 'Present address: Public Health Laboratory, 126, Mount Pleasant, countered in patients in the hospital between 1963 Liverpool, 3. and 1965. Received for publication 17 March 1967. Cross-infection between patients is clearly an im- 870 J Clin Pathol: first published as 10.1136/jcp.20.6.870 on 1 November 1967. Downloaded from Resistance to cloxacillin among hospital staphylococci 871 portant factor in the emergence in a hospital of anti- Rolinson, 1964; Parker and Jevons, 1964). Thus our biotic-resistant staphylococci. In an attemptto reduce cloxacillin-resistant cultures behaved as mixed popu- this at Sefton an isolation ward with single-room lations consisting of a majority of cells not a great accommodation for patients with staphylococcal deal more resistant to cloxacillin than cells of fully- sepsis was opened on 1 January 1964 (Turner, sensitive cultures and a small minority of cells able to Watson, and Abbott, 1965). Subsequently the major- grow in very high concentrations of the antibiotic. ity of patients found to have multiple-resistant staphylococcal infections were nursed in the isolation ward. As treatment with cloxacillin was largely con- DETECTION OF RESISTANCE TO CLOXACILLIN Cloxa- fined to patients with infections of this kind, that cillin-resistant cultures contain highly resistant cells antibiotic was used extensively in the isolation ward which are not only relatively few in number but also and seldom in the other wards of the hospital. The grow in vitro at a slower rate than sensitive cells; they ward is situated at one end of a single-storey block are as a result difficult to detect by the ordinary disc- which also contains six general medical wards; there diffusion technique on agar. is a separate nursing and domestic staff but it is We found, in disc-diffusion tests, that resistant necessary for night duty and holiday relief to be cultures showed with discs containing 5 ,ug. of cloxa- provided by staff from other wards. cillin or 10 ,ug. of methicillin (the concentrations commonly used) zones of inhibition which were re- METHOD OF INVESTIGATION duced in diameter by comparison with that obtained with sensitive cultures. This reduction clearly corre- During the three years, 1963 to 1965, a record was main- sponded to the slightly increased resistance of the tained of all isolations of coagulase-positive staphylococci majority of cells; but the difference in zone size was from infected lesions, nasal swabs, and other samples from not clear-cut because with both sensitive and resistant staff and patients in the hospital. Each culture was tested cultures it varied with inoculum size. On for sensitivity to antibiotics by the disc-diffusion method the other by inoculation on an agar plate of a 5 mm. loopful of a hand, withaheavy inoculum (e.g., the sensitivity plate flooded with an overnight broth culture) and after 48 suspension standardized to Brown's no. 1 opacity tube copyright. and application of discs containing penicillin (1 unit), hours' incubation, resistant cultures showed colonies streptomycin (10 jig.), tetracycline (25 uig.), erythromycin within the zone of inhibition up to the edge of the (5 ug.), fusidic acid (10 itg.), and cephaloridine (5 lAg.); the disc. depth of the agar in plates used for disc-diffusion tests was Better discrimination in terms of zone size was 3-4 mm. Tests for sensitivity to cloxacillin and methicillin obtained with a 1 pg. cloxacillin disc but 48 hours' were as described in the results section. During the period incubation was still necessary for the adequate under review, infections by cloxacillin-resistant staphylo- growth of resistant colonies. cocci were encountered in 39 patients and the strains from these were made the subject of special study. Cloxacillin- Altogether clearer definition of resistance was http://jcp.bmj.com/ resistant strains were also isolated from infections in two obtained when nutrient agar plates containing serial nurses and from nasal swabs from four patients not other- dilutions ofcloxacillin from 100,ug./ml. to 0-1 ug./ml. wise infected by a cloxacillin-resistant staphylococcus. were spot-inoculated with a 5 mm. loopful of an All multiple-resistant staphyloccoci were phage-typed. overnight broth culture ofeach staphylococcus under test. The results, examples of which are shown in RESULTS Table II, showed that the minimum inhibitory con- centration (M.I.C.) of cloxacillin was about 0-2 ,ug./ PATTERN OF RESISTANCE TO CLOXACILLIN All the ml., for penicillin-sensitive staphylococci, slightly on September 25, 2021 by guest. Protected cloxacillin-resistant staphylococci tested were also greater for those resistant to penicillin only, and up resistant to methicillin; and the pattern of resistance to 1 6 ,pg./ml. for multiple-resistant staphylococci; to cloxacillin was the same as that described in the these differences in M.I.C. among cloxacillin-sensitive case of methicillin by other workers (Sutherland and strains were presumably the result ofslow inactivation LE II GROWTH OF STAPHYLOCOCCI ON CLOXACILLIN AGAR Typical Strain Cloxacillin in Agar (p.g./ml.) 0-1 0-2 0-4 0-8 1-6 3 1 6-2 25 100 Penicillin-sensitive ++ - Resistant to penicillin only +++ ++ - - _ _ Multiple-resistant, cloxacillin-sensitive +±+ +++ ++ + _ _ Multiple-resistant, cloxacillin-resistant -1++ +++ +++ ++± ±±± ++ + 5 + + + - maximum confluent growth + + = numerous colonies + = 20-50 colonies 5 = 5 colonies. J Clin Pathol: first published as 10.1136/jcp.20.6.870 on 1 November 1967. Downloaded from 872 G. C. Turner and P. E. Cox of cloxacillin by penicillinase (Knox and Smith, 1963) frrom the infected lesion; this