APPLIED MicRoBIoLoGy, Feb. 1970, p. 345-352Copyright © 1970 American Society for Microbiology

Vol. 19, No. 2Printed in U.S.A.

Serratia marcescens: Biochemical, Serological, andEpidemiological Characteristics and Antibiotic

Susceptibility of Strains Isolated atBoston City Hospital

JAMES N. WILFERT,' FRED F. BARRETT,' W. H. EWING, MAXWELL FINLAND,AND EDWARD H. KASS

Channing Laboratory, Thorndike Memorial Laboratory, Harvard Medical Unit, and Department of MedicalMicrobiology, Boston City Hospital, the Department of Medicine, Harvard Medical School, Boston,

Massachusetts 02118, and the Enteric Bacteriology Laboratory, National CommunicableDisease Center, Atlanta, Georgia 30333

Received for publication 17 November 1969

The biochemical, serological, and epidemiological characteristics of 95 strainsof Serratia marcescens isolated at the Boston City Hospital were examined. Severalstrains were shown to be endemic, and the majority of isolates were cultured fromurine or respiratory secretions. Serratia species were highly resistant to polymyxinB and the cephalosporins, and various proportions were also resistant to otherantibiotics including kanamycin, but all of the isolates were sensitive to gentamicin.The appearance of resistance to kanamycin and nalidixic acid among endemicstrains was demonstrated. The nosocomial nature of Serratia infections, particu-larly those involving the urinary tract, was confirmed. Many clinical bacteriologylaboratories currently fail to identify the nonpigmented strains.

The problem of hospital-acquired infectionscontinues to be a major one. Gram-negativebacilli, particularly Klebsiella pneumoniae andPseudomonas aeruginosa, have been the most sig-nificant recently, but other species are also in-creasing in importance. In 1967 at the Boston CityHospital, an increasing incidence of isolations ofSerratia marcescens in the clinical bacteriologylaboratory was noted and led to the present study.

S. marcescens Bizio is a gram-negative bac-terium of the family Enterobacteriaceae and thetribe Klebsielleae. Some strains produce a redpigment, prodigiosin, and episodes dating back toantiquity of the "miraculous" appearance of"blood" on bread, communion wafers, and otherstarchy foods were probably from such Serratiastrains (15). In recent years, the organism hasbeen recognized as causing "pseudohemoptysis"(14, 26) and the "red diaper syndrome" (35).Serratia species were long considered a non-pathogen for humans, and pigment-producingstrains were used in the past as markers in ex-

' Present address: Division of Infectious Diseases, Departmentof Medicine, University of Utah College of Medicine, Salt LakeCity, Utah.

2Present address: Department of Pediatrics, Baylor Collegeof Medicine, Houston, Tex.

perimental work. However, in 1913, Serratia wasfirst reported as causing a pulmonary infection(38), and since then it was shown to be capable ofcausing suppurative infections of wounds, pneu-monia, lung abscess, empyema, meningitis,urinary tract infections, endocarditis, septicarthritis, osteomyelitis, peritonitis, sinusitis, andsepticemia. The literature on Serratia infectionswas reviewed recently by Ringrose et al. (25).

It has also been observed that nonpigmentedstrains of Serratia were more common than pig-mented ones (5, 10) and that many clinical bac-teriology laboratories were unable to identifynonpigmented strains correctly (13). In 1962,Ewing et al. (11) pointed out the nosocomialnature of many Serratia infections. Several hos-pital outbreaks involving urinary tract infections(1, 4, 18, 22, 31, 36) and respiratory tract infec-tions (2, 3, 25) and two epidemics in nurseries fornewborn infants (21, 30) have been described.Infections also have been noted to occur at thesite of indwelling intravenous catheters (7, 23,37) and after lumbar punctures (16, 32) or peri-toneal dialysis (21). Previous antibiotic therapyand underlying chronic debilitating disease mayalso predispose to serious Serratia infection (7,

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346 WILFERT ET AL.

37). Treatment of these infections may be a diffi-cult problem because many recently isolatedstrains of this organism have been resistant tomany antibiotics, although most isolates havebeen sensitive to kanamycin and all to gentamicin(27, 33, 37).The present study deals with the isolation,

identification, and some of the epidemiologicalfeatures of the Serratia strains isolated at theBoston City Hospital.

MATERIALS AND METHODS

During the 5 months, September 1967 throughJanuary 1968, all isolates of S. marcescens obtainedfrom cultures of patients in the outpatient department,accident floor, and hospital wards were collected.Multiple isolates from the same source in any patientwere usually not collected, but an effort was made toobtain isolates from various sites in the same patient.

Initial identification was made in the BacteriologyLaboratory under the supervision of A. Kathleen Dalyand Alice McDonald. Serratia strains were identifiedby previously described methods used in this labora-tory (8, 12) and confirmed at the Enteric BacteriologyLaboratory of the Communicable Disease Centerwhich also carried out serological identification (8).

Antibiotic sensitivity tests were done by the inocula-replicating method of Steers et al. (29). The 111 strainstested included the original isolates plus the isolatesfrom different sites in these same patients. An over-night culture in Brain Heart Infusion Broth (Difco)was diluted 10(4 and replicated on Beef Heart In-fusion Agar (Difco) containing serial twofold dilu-tions of the antibiotics. The antibiotics used and theirsuppliers were: streptomycin sulfate (E. R. Squibb &Sons); sodium cephalothin, cephaloridine, cephalexin,and cephaloglycin (Eli Lilly & Co.); sodium ampicil-lin and kanamycin sulfate (Bristol Laboratories Div.);tetracycline hydrochloride (Lederle Laboratories);chloramphenicol (Parke, Davis & Co.); polymyxin Bsulfate (Burroughs Wellcome & Co.); gentamicin sul-fate (Schering Corp.); rifampin (Ciba Corp.); andcarbenicillin (Beecham Products Inc.).

Clinical and epidemiological data were obtainedfrom the patients' hospital records. The informationsought included temporal sequences in relation tohospitalization, prior antibiotic therapy, results of allbacteriological cultures, severity of the illness, under-lying diseases and all prior surgical procedures, ormanipulations of the genitourinary or respiratorytract. An organism was considered to be hospital-acquired when previous cultures were negative forSerratia, and the clinical significance of the isolateswas based on the presence of signs of clinical infectionwhen Serratia was the only or predominant organism.Bacteriuria was considered to be significant only whenthe colony counts of urine cultures were > 105/ml(17). Isolates were streaked and stabbed on TrypticaseSoy Agar slants in tubes (13 by 100 mm), and thensealed with paraffin-impregnated corks and stored atroom temperature in the dark.

APPL. MICROBIOL.

TABLE 1. Biochemical reactions of 95 isolates ofSerratia marcescens

Per cent positiveTest or substrate Per cent

negative1-2 Days 3-14 Days

Hydrogen sulfide (TSI) 100Voges-Proskauer (37 C) 99 1Indole 1 (week) 99Citrate (Simmon's) 100Motility 100Gelatin (22 C) 90.5 9.5Lysine decarboxylase 100Ornithine decarboxylase 100Glucose: acid 100Glucose: gasa 55.8 44.2Lactose 100Dulcitol 100Adonitol: acid 83.1 7.4 9.5Adonitol: gas 100Inositol: acid 94.7 2.1 3.2

100Arabinose 100Raffinose 100Rhamnose 100Lipase (corn oil) 97.9 2.1Cellobiose: acid 9.5 16.9 73.6Cellobiose: gas 100Glycerol: acid 99 1Glycerol: gas 100Deoxyribonuclease 92.4 7.6Pigment 7.4 92.6

a Less than 10% volume.

RESULTSBiochemical reactions. The biochemical reac-

tions of S. marcescens and its differentiationfrom other organisms have been reported in de-tail elsewhere (6, 8, 9). The reactions of the 95primary isolates from this study are given inTable 1. It will be seen that these strains werehighly consistent in most of their reactions, andreactions were complete in 1 to 2 days in most in-stances. At this hospital, the reactions most help-ful for identifying Serratia species have been theconsistent inability to ferment lactose, positivelysine decarboxylase, and the inability of Serratiato form more than small amounts (< 10% in aDurham tube) of gas with glucose as a substrate.The only subspecies, S. marcescens var. kiliensis,differs from the type species S. marcescens onlyin that it is Voges-Proskauer negative. Only onesuch strain was found. Pigment was produced byonly 7.4% of strains isolated.

Serological studies. To date, 15 somatic ("O")and 13 flagellar ("H") antigens have been estab-lished for Serratia. Difficulty in serotyping be-cause of "roughness" has not been a major prob-lem; the majority of strains from a variety ofsources and localities are typable. There were 23

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TABLE 2. Distribution of serotypes of Serratia marcescenis

H anti-ensb0 antigensa

.-

2

45

67911121314

0 undeterminedRoughTotal

173

21

2

1

2

3

1

1

4

211

2

10

1

1

37

5

1

1

7 8 11

1

1

a There were no serotypes of 0 antigens 3, 8, 10, and 15.b There were no serotypes of H antigens 6, 9, and 10.

2

2

2

1

1

5

H unde-12 13 termined

1

4 I

5 3

3 1i 5

3 13 11 11

1001

90 ,LD801

0 50

30-3

~ ~ ~ ~ ~ ~

20

0.1 0.2 0.4 0.8 1.6 3.1 6.3 12.5 25 50 100 >100MINIMUM INHIBITING CONCENTRATION, MICROGM. PER ML.

FIG. 1. Susceptibility of Serratia marcescens toeight antibiotics: 111 strains isolated at Boston CityHospital, September 1967 through January 1968. Allstrains were resistant to 100 ,ug/ml of cephalothin andpolymyxin B (not depicted in this figure).

different serotypes among the present isolates(Table 2). The 0 antigens of 8 strains were notidentifiable with available antisera, and the Hantigens of 11 were undetermined. For ninestrains only the 0 or the H antigen was identifi-able; nine strains were completely untypable, butonly one strain was rough. The four most frequentserotypes were 02:H4 (22%), 04:Hl (18%),011 :H4 (11%), and 011 :H13 (5%).

In most cases, multiple isolates from the samesource or isolates from different sources in thesame patient were not serotyped separately. How-ever, in those cases in which multiple isolates were

serotyped, the type was the same; these cases arenot included in the analysis. In one culture fromthe stool of an infant in the outpatient depart-ment, two serotypes were identified. Three of theeight pigmented strains were of the same serotype.

Antibiotic susceptibility. In addition to theantibiotics shown in Fig. 1, all strains were testedwith cephalothin and polymyxin B, but none wasinhibited by a concentration of 100 ,ug/ml ofeither antibiotic. Nine strains were tested againstrifampin, cephalexin, cephaloglycin, and carbeni-cillin. All were resistant to 100 ug/ml of cepha-lexin and cephaloglycin. Rifampin inhibited sixstrains at a concentration of 50 ,ug/ml and threeat 100 ,ug/ml; carbenicillin inhibited one strainat 25 Ag/ml and one at 200 ,ug/ml, but the otherseven strains grew well in the latter concentration.By using a tube-dilution method, Thornton andAndriole (33) found that 8 of 35 isolates of Serra-lia had a minimal bactericidal concentration of50 ,ug/ml or less against carbenicillin and sug-gested that this antibiotic might be of use in thetreatment of urinary tract infections. Nitro-furantoin was not tested against the presentstrains because standard discs containing 100 Mghave consistently shown no zone of inhibition.As previously noted (37), nearly all Serratia

strains are susceptible to gentamicin; all but 2 ofthe 111 isolates tested in this study were inhibitedby 6.3 ,ug/ml or less (within the safe and nontoxicrange for gentamicin serum levels). Nalidixic acidwas inhibitory at 3.1 ,g/ml for about 80% of thestrains, and the rest required much higher con-centrations. Thirty-two per cent of the isolateswere resistant to 25 Mg of kanamycin per ml, and

Total

1251993I2

1811681

95

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TABLE 3. Sources of Serratia isolates

Other sources in same patientsSource isolates

Urine Sputuma Wound Blood

Urine 34 1 4 3Sputum 48 3 1Wound 7Blood 3 1 1 1Feces 2 1a

a Includes one isolate from nasopharnyx.

0.1 0.2 0.4 0.8 1.6 3.2 6.4 12.5 25 50 100 >100

MINIMUM INHIBITING CONCENTRATION, MICROGM. PER ML.

FIG. 2. Susceptibility of two groups of strains ofSerratia. Comparison of the 111 isolates in this studywith 17 strains received 10 years earlier at the Com-municable Disease Center.

551%1 were resistant to this concentration of strep-tomycin. As can be seen from Fig. 1, the otherantibiotics were even less active.

In an effort to ascertain whether drug resistancehad developed or increased in Serratia speciesover the past 10 years, tests were done with 17strains which were sent to the CommunicableDisease Center in 1957-58 from various labora-tories and used there as the type strains for pre-paring the original antisera for typing. The sus-ceptibility of these 17 strains to those antibioticsfor which changes were noted is compared withthat of the strains isolated in the present study(Fig. 2). All of the earlier strains were susceptibleto kanamycin and nalidixic acid, neither of whichwas in use in this country at that time. Further-more, 18 of the 25 recent strains that were foundto be resistant to kanamycin were of the 4 sero-types that are endemic in this hospital. In thecase of nalidixic acid, 11 of the 14 strains thatwere not inhibited by 25 Ag/ml were of the same4 serotypes; all but one of these strains were iso-lated from urine. Kanamycin was used frequentlyin this hospital, and the appearance of resistancewas not unexpected in endemic strains. Nalidixicacid, on the other hand, was infrequently usedhere from 1964 through 1967, and resistance to itcannot be explained on this basis.

Epidemiologic observations. The sources of the94 cultures that yielded Serratia species and theother sites with positive cultures in the same pa-tients are shown in Table 3, and Table 4 gives theincidence of hospital acquisition and the estimateof the clinical significance of the strains accordingto their source. The most common sources wereurine and sputum (including specimens obtained

TABLE 4. Acquisition anzd clinical significanice ofSerratia isolates

Hospital-acquired ClinicallysignificantSource No. ofSore isolatesa

No. Per cent No. Per cent

Urine 38 25 66 26 68Sputum 52 24 47 3 6Woundsb 13 11 85 4 36Blood 6 6 100 5Feces 2 0 0

a Includes secondary sources.b Includes venous catheters.

by endotracheal suction, via a tracheostomy tube,or nasopharyngeal swabs). Patients with urinaryisolates rarely had the organism in their sputumand vice versa. Serratia species were rarely iso-lated from stool specimens; the two strains in thepresent study were from infants. One of these twostrains was pigmented, and the same serotype wasalso present in the nasopharynx of the sameinfant.

Bacteremia resulting from S. marcescens in pa-tients at this hospital has previously been re-ported (37). In the present survey, there are threeprimary and three secondary isolations of Serratiaspecies from blood cultures. In five of these sixcases, the portal of entry was the urinary tract(four cases) and an indwelling venous catheter inone; in the sixth, blood cultures were drawnthrough an indwelling venous catheter and yieldedthree different organisms in addition to Serratiastrains, so that contamination may have ac-counted for that isolation.Of the 13 isolations of S. marcescens from

wound cultures, 3 were cultured directly from in-dwelling venous catheters, and one was from a"scalp vein needle" withdrawn from a veinaround which there was a cellulitis. The woundsfrom which three other isolates were grown fol-lowed abdominal or genitourinary surgery, andcultures of the urine in these cases also yielded

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TABLE 5. Distribution of the more frequent serotypes according to source of specimenand location of patientP

Source Location

Serotype Genitour- Outpa- Medical ward

Sputum Urine Wound Blood inary tient de- Neu-__-olg Othersward partment AolBgC

02:H4 1 18 2 16 1 2 204:H1 12 1 3 1 4 4 9011:H4 8 2 3 3 4011:H13 4 1 1 3 1

a Values indicate number of patients.

TABLE 6. Factors associated with isolation of Serratia strains

Source No. of strains Associated conditions No. of strains Per cent of source

Urine 38 Indwelling catheter 28 74Urological surgery 15 39Cystoscopy 13 13One or more of the above 32 84None of the above 6 16

Sputum 51 Intermittent positive pressure breathing 12 24Nebulizer 16 31Intubation 15 29One or more of the above 26 51None of the above 25 49

Serratia species. The other wounds followed sur-gical procedures except one from a case of cellu-litis acquired outside the hospital.The data in Table 5 suggest that strains of the

four most frequently isolated serotypes werenosocomial. Of the 21 isolates of 02:H4, 8 werefrom urine; three of the patients with positiveblood cultures also had this serotype in theirurine. Each of these 21 patients had acquired theorganism in the hospital or outpatient depart-ment; 16 patients were on the male genitourinaryward, and one patient, who was hospitalized hereprior to the present study, was in the urologicaloutpatient department. Serotype 011:H13 wasisolated four times from urine of urological pa-tients and once from a case of nonhospital-ac-quired cellulitis.The strains with serotypes 04:H1 and 011 :H4

were primarily from respiratory sources; at least70% of 04:H1 isolates were hospital-acquired.Almost half of the 04:H1 strains were from twowards, one medical and the other neurological-neurosurgical. Fewer of the 011 :H4 strains werehospital-acquired and two medical wards wereeach the source of three of them.

Table 6 lists the procedures associated withcolonization or infection in patients who yielded

Serratia species from urine and sputum. Of thepatients with Serratia strains in their urine, 84%had some form of urological manipulation, in-dwelling catheterization being the most common,and 80% had received antibiotics. In three casesin which there was no previous manipulation, theSerratia strains were probably contaminants.

In two previously described hospital outbreaksof Serratia species from respiratory sources, thecommon source was found to be contaminatedintermittent positive pressure breathing machines(IPPB; 3) or ultrasonic nebulizers (25). In thepresent study, 26 of 52 patients with respiratoryisolates of Serratia had been subjected to IPPB,nebulization, or some form of endotrachealmanipulation; this occurred in 91% of thoseharboring serotype 04:H1. Antibiotics had beengiven to 64% of these patients. The seven pig-mented strains all were isolated from respiratorysources and were also cultured from feces in onepatient and from urine in another.

Clinical observations. The mean age of patientsin this study was 61 years, excluding one newborninfant and two infants under 1 year old; 79% weremales. Possible factors associated with Serratiacolonization or infection were sought. None ofthe patients had received corticosteroids or anti-

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metabolites before the first positive culture forSerratia, but 7 had diabetes, 23 were chronic alco-holics, 14 had some malignancy, and 17 were inthe early postoperative period. Most of the post-operative isolates followed prostatic surgery.

DISCUSSIONThe urinary tract has been the most frequent

site of Serratia infections. The epidemiology ofhospital outbreaks of Serratia infections of theurinary tract is still unclear. In an outbreak in-volving 135 cases of urinary tract infections re-sulting from Serratia on the "service" wards (incontrast to wards for private patients), Allen andConger (1) confirmed the high incidence of in-dwelling catheterization and urinary tract ab-normality noted by others. Their epidemiologicalstudy yielded negative results, except for the pres-ence of Serratia species on the floor beneath thebeds of patients with persistently positive urinecultures. Serotyping of strains was not done, butthe uniformity of the antibiotic sensitivity datasuggests that a single strain or a small number ofstrains were involved. Overcrowding was a sig-nificant factor, and the suggestion was made thatclosed drainage systems for all catheters, twicedaily mopping of the floor with germicidal solu-tions, and isolation of infected patients might beuseful in preventing or combating such outbreaks.Wheat et al. (36) reported 11 patients with

urinary infections from pigmented strains andnoted that all had undergone instrumentation ofthe urinary tract. An attempt to determine thesource of the organism was unrevealing; sero-typing was not done. Magnuson and Elston (22)reported seven cases of Serratia infection, five ofwhich were in the urinary tract. Two of the uri-nary strains were typed as 05:H1, and a woundinfection also yielded this serotype. In anothercase, Serratia 05:H10 was isolated from sputumand urine and also from floor dust adjacent to thepatient's bed.

In a series of 104 patients from whom Serratiawas isolated from various sites over one year,Clayton and von Graevenitz (4) noted six cases,including four urinary tract infections, whichoccurred within 5 days on one ward. In the surveyreported by Ewing et al. (11) of the serotypes ofSerratia from three hospitals, 19 strains in onehospital were isolated from urine and three sero-types predominated, 04:H4, 013 :H7, and014:H4. Lancaster (18) reported 20 cases ofSerratia urinary tract infections which occurredin 10 months; eight of the strains were serotypedbut were heterogenous. Detailed epidemiologicaldata were not reported; all but one of the patientshad been catheterized. Taylor and Keane (31)described an outbreak of seven cases on a uro-

logical ward from a pigmented strain. All patientshad indwelling catheters, and the outbreak wasended by stricter attention to care of the drainagesystems. Environmental cultures were negativeand serotyping was not done.

In the present study, strains of serotypes 02:H4and 011 :H13 were associated with urinary tractinfections acquired mainly in the male urologicalwards and outpatient clinic. Interestingly, 02 :H4strains were frequently resistant to kanamycin,whereas 011:H13 strains were not. The expectedhigh incidence of urinary tract instrumentationwas found, but a point source was never identi-fied. Environmental cultues were not done.When Serratia strains are found in the urine, it

is usually of clinical significance. In all of the casesreported by Allen and Conger (1), colony countswere high; this was true in 68% of our cases. How-ever, the organism may appear as a contaminant,and, when this is suspected, cultures should berepeated.Four of the 26 patients with "significant" bac-

teriuria developed bacteremias. This high ratemay be attributable to the fact that many of thesepatients had been recently subjected to prostaticsurgery, the fresh wound forming an easy portalof entry.The treatment of urinary tract infections in pa-

tients with indwelling catheters or after prostaticsurgery is difficult. For Serratia species, the pre-sumptive drug of choice currently is gentamicin,unless sensitivity tests show the organism issensitive to a less toxic agent. For treatment ofasymptomatic bacteriuria due to Serratia whenall catheters have been removed and surgicalwounds are healed, nalidixic acid may be usefuland perhaps should be tried even when disc sensi-tivity studies show resistance, since drug levels inthe urine may reach 50 to 200 Ag/ml (19).Although the most frequent source of Serratia

isolates in this study was the respiratory tract,these isolations were infrequently of clinical sig-nificance (Table 4). In neither of the Serratiaoutbreaks associated with respiratory equipmentwas the incidence of infection, as opposed tocolonization, reported. However, Serratia speciescan cause pneumonia, empyema, and lung ab-scess. In the two largest series of Serratia bac-teremias reported, roughly 15% had the respira-tory tract as the portal of entry. Tillotson andFinland (34) reported 149 pneumonia cases inwhich serial cultures were made. Serratia strainswere not isolated from the initial culture in any ofthese cases. However, this organism was subse-quently isolated from 10 of the patients, in pureculture in 4 of them, but in no instance did super-infection (in contrast to colonization) by thisorganism ensue.

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The findings in this study seem to confirm therole of respiratory equipment and proceduressuch as endotracheal suction in the spread ofSerratia species in a hospital. Cabrera (3) showedhow a respiratory strain could infect wounds andthe urinary tract by demonstrating contaminationof containers of "sterile" saline used for irrigationof wounds and catheters and findings that suchcontainers were also used to fill intermittent posi-tive pressure breathing machines which wereprobably contaminated in this manner. In thepresent study, intensive epidemiological studies ofrespiratory equipment were not done since therespiratory strains did not present a clinicalproblem. Both previous studies pointed out thedifficulty in sterilizing respiratory equipment oncecontamination occurs. Ringrose et al. (25) had todiscontinue the use of ultrasonic nebulizers toend the outbreak. Cabrera (3) found that only bydiscontinuing use of reservoir nebulizers in theIPPB machines and sterilizing the "medicationnebulizer," manifold tubing, and mouthpiece be-tween each patient usage could the outbreak becontrolled.The frequency with which the respiratory tract

was the source of the pigmented strains is of in-terest. It is not known whether this representssome propensity on the part of pigmented strainsto grow in the respiratory tract or merely reflectsthe higher number of serotypes and non-hospitalacquired strains among the respiratory isolates.

Other types of hospital outbreaks and iatro-genic infections from Serratia species deservecomment. Rabinowitz and Schiffrin (24) de-scribed an outbreak of 11 cases of meningitis,wound infections, or arthritis on a pediatric ward.Serratia was isolated from a single bottle of 5%dextrose in saline used intravenously and topi-cally in several of the cases. In an outbreak in anursery of newborn infants reported by McCor-mack and Kunin (20), the reservoir of infectionwas found to be contaminated caps of "cordsaline" bottles. Serratia species were isolated from20 newborns, and in 22% of nursing mothers thebreasts were colonized. Eight cases of clinical ill-ness were associated with S. marcescens amongthese patients (five cases of urinary tract infec-tions, and one each of balanitis, omphalitis, andrespiratory tract infection). The strain was a pig-mented one, 06:H2. Stenderup et al. (30) re-ported on a more serious epidemic in a maternityhospital affecting premature infants. There weresix deaths due to septicemia, six cases of purulentconjunctivitis, and one case of arthritis withpossible septicemia. The strain in these cases wasnonpigmented, 014 :H12. An epidemiologicalsurvey was unrevealing.

Tlhe iatrogenic nature of other types of Serratia

infection is illustrated in several reports. The caseof meningitis in the outbreak reported by Rabino-witz and Schiffrin (24) was from intravenous in-jection of a contaminated solution. In two cases,meningitis followed lumbar puncture for diag-nostic purposes (32) or spinal anesthesia (16).Both organisms were pigmented strains; one wasserotype 05:H2. Shedden (28) described threecases of meningitis and septicemia which followedinsertion of Holter valves for hydrocephalusassociated with spina bifida. This cluster of threecases suggests some common source. McCrackenand Lipscomb (21) reported three cases of peri-tonitis associated with peritoneal dialysis; the iso-lates were all pigmented, and two of the cases oc-curred simultaneously. The association of Serra-tia infections with indwelling venous cathetersnoted previously (37) was confirmed in the pres-ent study.The development of hospital-acquired anti-

biotic resistance is well illustrated in the presentstudy. Although all of the 1957-58 strains werekanamycin-susceptible, 35% of the isolates in thepresent study, most of them of endemic serotypes,were resistant.The true dimensions of the problem of noso-

comial infections from Serratia species will notbe known until most clinical bacteriology labora-tories are able to identify the nonpigmentedstrains. In September 1968, the Diagnostic Labo-ratory of the Massachusetts Department of PublicHealth sent out a typical strain of S. marcescensto the 121 laboratories which participate in itsLaboratory Approval Program; 60% of themwere unable to identify the organism correctly.It was most frequently called Enterobacter orPseudomonas species, but was also identified asShigella, Proteus, Providencia, or Escherichia coli.Recent reports of series of clinical isolates showthat many laboratories are still reporting speciesas "Paracolon," "intermediate coliforms," or"Paracolobactrum" species, and strains ofSerratia may well be included in these groups.The therapeutical and epidemiological implica-tions of a Serratia infection demand its promptidentification. It should be emphasized thatSerratia strains can be readily identified 1 to 3days after isolation by using commonly availablemedia.

ACKNOWLEDGMENTS

The authors are grateful to the staff of the Bacteriology Lab-oratory of the Boston City Hospital for their cooperation, toBetty Davis for performing the serotyping, and to Clare Wilcoxfor assisting with the susceptibility tests.

This study was supported by grant HD 03693 from the Na-tional Institute of Child Health and Human Development andgrants 5R1-A1-0023 and 2T1-AI-00068 from the National In-stitute of Allergy and Infectious Diseases. F. F. Barrett wasEpidemic Intelligence Officer, Epidemiology Program, NationalCommunicable Disease Center.

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LIRATURE CITED

1. Allen, S. D., and K. B. Conger. 1969. Serratia marcescensinfection of the urinary tract: a nosocomial infection. J.Urol. 101:621-623.

2. Burdin, J. C., J. Lochard, F. Schaak, M. Edert, and J-C.Georges. 1967. Epidemic d'infections pleuro-pulmonariesdues a Serratia marcescens en milieu hospitalier. PresseMed. 75:466-468.

3. Cabrera, H. A. 1969. An outbreak of Serratia marcescensand its control. Arch. Intern. Med. 123:650-655.

4. Clayton, E., and A. von Graevenitz. 1966. NonpigmentedSerratia marcescens. J. Amer. Med. Ass. 197:1059-1064.

5. Davis, B. R., and W. H. Ewing. 1957. The biochemical re-actions given by members of the Serratia group. Int. Bull.Bacteriol. Nomencl. Taxon. 7:151-160.

6. Davis, B. R., and W. H. Ewing. 1964. Lipolytic, pectinolytic,and alginolytic activities of Enterobacteriaceae. J. Bacteriol.88:16-19.

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