APPLIED MICROBIOLOGY, Aug. 1969, p. 198-203Copyright © 1969 American Society for Microbiology

Vol. 18, No. 2Printed in U.S.A.

Klebsiella, Enterobacter, and Serratia: BiochemicalDifferentiation and Susceptibility to Ampicillin

and Three Cephalosporin DerivativesRONALD J. ZABRANSKY, JACK W. HALL, FRED E. DAY, AND GERALD M. NEEDHAM

Mayo Clinic and Mayo Foundation, Rochester, Minnesota 55901

Received for publication 3 April 1969

Three hundred twenty-nine strains of the tribe Klebsielleae were compared byseveral biochemical tests and by susceptibility to selected antibiotics. Biochemicaltests included urease, amino acid decarboxylase, and hydrogen sulfide production;fermentation of lactose and dextrose; motility; and tests in the IMViC (indole,methyl red, Voges-Proskauer, citrate) series. The isolates were: Klebsiella species,67.5%; Enterobacter species, 28 %, and Serratia species, 4.5 %. Minimal inhibitoryconcentrations of cephaloridine, cephalothin, and a new cephalosporin, cephalexin,and of ampicillin were determined by the agar dilution procedure. Cephalosporinsat 20 ug/ml or less inhibited 90% of the Klebsiella strains but only 15% of theEnterobacter strains. Ampicillin inhibited 27% of Enterobacter strains and 17% ofKiebsiella strains. Serratia isolates were insensitive to the cephalosporins and ampi-cillin. The results suggest that precise identification of this group to the generic levelcan be accomplished readily in the clinical laboratory and that such information ishelpful in the preliminary selection of an antibiotic for treatment of clinicalinfections.

The recent change in nomenclature of orga-nisms in the tribe Klebsielleae (10) [Klebsiella,Enterobacter, and Serratia; 5, 11, 13], the in-creasing importance of gram-negative bacteriaas causes of clinically important infections, andthe conflicting reports pertaining to the sus-ceptibility of organisms of this group (2, 8, 16,17) to certain antibiotics prompted us to evaluatethe biochemical characteristics and the anti-biotic susceptibility of members of the tribeKlebsielleae isolated in the Mayo Clinic labora-tories.For many years the appearance of the colonies

of the lactose-fermenting Enterobacteriaceaeon eosin-methylene blue-agar has been a primarybasis for identifying and separating the "colon-aerogenes" group. This method readily dis-tinguishes Escherichia coil from the Klebsiella-Enterobacter-Serratia group of bacteria with ahigh degree of accuracy, but it provides no meansof identifying atypical strains and it fosters thelumping of organisms into a broad, ill-definedgroup. The suggestions of Lampe (18) andThaler (22), that these organisms cause similarinfections requiring similar treatment and there-fore the use of complicated and lengthy labora-tory procedures for separation and differentia-

tion is not warranted, are no longer acceptable.The introduction of new antibiotics and chemo-therapeutic agents with a wide range of effective-ness against certain gram-negative bacteria,particularly ampicillin and the cephalosporins,has made it necessary to identify infectingorganisms as precisely as possible. For example,ampicillin is usually effective against E. coil andProteus mirabilis strains but not against Kleb-siella, Enterobacter, Serratia, or Providenciaspecies (1). Similarly, the report by Fleming andassociates (15), that Klebsiella species are sensi-tive to cephalothin but Aerobacter (Enterobacter)species are not (because of the production of acephalosporinase by the latter), suggests that it isimportant for the clinical laboratory to provideaccurate bacterial identification.Our study was undertaken to determine (i)

the frequency of isolation and (ii) the suscepti-bility to selected antibiotics of members of thetribe Klebsielleae in material presented to theclinical laboratory of the Mayo Clinic. In addi-tion, biochemical tests used for the classificationwere evaluated in the clinical laboratory setting.The primary group of antibiotics chosen for thisstudy were the cephalosporins, of which one hasgiven conflicting results (2, 4, 6, 8, 16, 17, 20).

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The new orally administered derivative, cepha-lexin, and cephaloridine were included to de-termine whether they exhibit any differences inactivity against these organisms. Ampicillin wasadded to determine whether its effectiveness wasuniform for this group when tested under thesame circumstances as the cephalosporins. Otherantibiotics which may be more effective in thetreatment of clinical infections were not includedbecause this was not the purpose of the study.

MATERIALS AND METHODSIsolates (352) from clinical material (urine, sputum,

blood, wound swabs, and swabs, tissue, or fluids frommiscellaneous infections) were selected initially bygross colonial morphology on eosin-methylene blueand blood-agar plates. The reactions of the isolates inselected fermentative, biochemical, and motility testswere then determined. Basic tests were: fermentationof lactose and dextrose (triple sugar-iron-agar orpeptone water with 1% carbohydrate); production ofindole (nutrient broth with 0.5% additional peptoneand tested with Kovacs' reagent); acetoin (methylred-Voges-Proskauer medium); hydrogen sulfide(triple sugar-iron-agar); urease (Christensen's urea-agar); ornithine and lysine decarboxylase production(Moeller's decarboxylase medium with 0.3% agar and2% amino acid); acidity with methyl red (methyl red-Voges-Proskauer medium); motility (motility agar,0.3%); and citrate utilization (Simmons' citrate-agar).Tubes were incubated overnight at 37 C (motilitymedium was incubated at 22 C), and all reactionswere read at 18 to 24 hr, except the methyl red-Voges-Proskauer test and the amino acid decarboxylasemedia, which were read at 48 and again at 72 hr. If thereactions in motility agar and citrate-agar were notdecisive at the end of 24 hr, these tests were reincu-bated and read again at 48 hr.When atypical results were obtained, the tests were

repeated and certain alternative procedures were

attempted, particularly motility studies at differentincubation temperatures and in liquid media (hanging-drop preparation). Determination of lysine decar-boxylase or deaminase and hydrogen sulfide produc-tion was done on lysine-iron-agar in a few instances.Organisms that remained difficult to classify wereclassified "atypical" and were tested for fermentationof sucrose, rhamnose, arabinose, raffinose, andsorbitol and for production of arginine dihydrolaseand phenylalanine deaminase.

The minimal inhibitory concentration (MIC) ofampicillin and three cephalosporin antibiotics (cephal-othin, cephaloridine, and cephalexin) for the isolateswas determined by the agar dilution procedure (9)with the Steers et al. (21) replicator. Dilutions of eachantibiotic at 5, 10, 20, 50, 100, and 200 ,ug/ml wereprepared in Trypticase Soy Agar. Six-hour nutrientbroth cultures were used as inocula and the plateswere examined after 18 hr of incubation at 37 C.The MIC was recorded as that concentration produc-ing complete inhibition of growth.

RESULTS

Biochemical tests. Quick and accurate identi-fication of groups of organisms by colonialmorphology has long been recognized by ex-perienced bacteriologists (3, 14, 19) as havingcertain practical advantages. In our experience,the selection of isolates solely on the basis ofgross colonial morphology in eosin-methyleneblue and blood-agar was accurate 94% of thetime in identifying members of the Klebsiella-Enterobacter-Serratia group. Only 23 of theoriginal 352 isolates were found not to be KMb-siella, Enterobacter, or Serratia. These 23 strainswere subsequently identified as E. coli (17 strains),Providencia (2 strains), and Citrobacter (4strains) and were excluded from this study.The descriptions of the species of Klebsiella,

Enterobacter, and Serratia by Edwards andEwing (7), Ewing (11), and Fife et al. (13) wereused for classification. The distribution andsource of the 329 strains of the tribe Klebsielleaein our study are presented in Table 1. Klebsiellaisolates were found more than twice as often asEnterobacter. Serratia isolates were found in-frequently, in agreement with the results ofLerner and Weinstein (20).Table 2 shows the distribution of the positive

reactions in the basic biochemical tests. Twenty-eight Klebsiella strains (13%) varied from thetypical or definitive reactions of the genus (in-dole-negative, citrate-positive, ornithine-decar-boxylase-negative, lysine-decarboxylase-positive,nonmotile). The significant variances were inthe results for indole production (3%), citrate

TABLE 1. Distribution and source of 329 strains of tribe Klebsielleae

Source

Genus Total no. Per cent of total AbscessUrine Sputum wounds, Blood Other

etc.

Klebsiella ............... 222 67.5 81 58 41 28 14Enterobacter ............ 92 28.0 30 22 20 12 8Serratia ................ 15 4.5 4 5 5 0 1

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utilization (6%), and lysine-decarboxylase pro-duction (7%), but these variances are withinthe limits reported by Fife et al. (13).Twenty of the 92 strains of Enterobacter (22 %)

varied from typical or definitive reactions of thegenus (indole-negative, citrate-positive, ornithine-decarboxylase-positive, motile). The significantvariances were with strains that were eitherornithine-decarboxylase-negative (8%) or non-motile (12%). The majority of these strains wereidentified as either E. liquefaciens or E. hafniaeby use of additional tests. Atypical results arecommon among these two species, and compari-son of the results of reactions at 22 and 37 Cwas frequently helpful in separation and identifi-cation of these organisms.The results of the biochemical tests of the 15

strains of Serratia are similar to those reportedin the literature, even though only a small numberof strains were examined. All strains were incu-bated for 3 days at 22 C on nutrient agar forpigment production, but only two strains pro-duced prodigiosin. None of these strains fer-mented rhamnose, raffinose, or arabinose.

Antibiotic sensitivity. Table 3 shows thedistribution of MIC for the four antibioticstested. Of the Klebsiella strains, 57% or greaterwere inhibited by each of the three cephalo-sporins at 5 ,g/ml. Most of the remaining strainswere inhibited by either 10 or 20 jig/ml of thesesame antibiotics. Higher concentrations ofampicillin were needed to inhibit the Klebsiellastrains, and greater variability in antibioticsusceptibility was noted. Only 37 strains (17%)

were inhibited by 20 Ag/ml or less, and 64 strains(29%) were not inhibited by 200 ,ug/ml.The Enterobacter strains exhibited the same

susceptibility pattern to ampicillin as did theKlebsiella strains, 27% of the strains being in-hibited by concentrations of 20 Ag/ml or less.However, these strains differed markedly fromthe Klebsiella strains in susceptibility to thecephalosporins. Only six (7%) Enterobacterstrains were inhibited by any of the three cephalo-sporin antibiotics at 5 ,ug/ml, and large numbers

TABLE 2. Biochemical reactions of 329 strains oftribe Klebsielleae

Positive reactions

Test Klebsiella Enterobacter Serratia(222 strains) (92 strains) (15 strains)

No. % No. % No. %

Lactose ......... 218 98 81 88 3 20Dextrose ........ 222 100 92 100 15 100Indole........... 7 3 1 1 0 0Methyl red ...... 15 7 3 3 0 0Voges-Proskauer. 203 92 89 97 14 94Citrate .......... 209 94 91 99 15 100Lysine decarbox-

ylase .......... 206 93 33 36 15 100Ornithine decar-

boxylase. ........ 0 0 85 92 13 87Motility ......... 0 0 81 88 15 100H2S.0 0 0 0 0 0Urease .......... 199 90 50 54 3 20

TABLE 3. Numbers of strains inhibited at various concentrations of antibiotics

Antibiotic concn (jug/ml)Antibiotic

5 10 20 50 100 200 >200

Klebsiella strains (total222)

Ampicillin. 4 2 31 23 38 60 64Cephalothin .......... 127 43 28 14 3 1 6Cephaloridine.146 30 16 13 2 2 13Cephalexin.. 142 52 13 4 2 2 7

Enterobacter strains(total 92)

Ampicillin. 1 3 21 10 19 13 25Cephalothin.6 3 2 2 3 5 71Cephaloridine......... 6 3 6 1 0 0 76Cephalexin.. 6 7 1 9 8 18 43

Serratia strains (total15)

Ampicillin.0 0 2 4 7 2 0Cephalothin.. 0 0 0 0 0 0 15Cephaloridine 0 0 0 1 0 0 14Cephalexin .0 0 0 1 0 0 14

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of the strains (47 to 83%) were not inhibited byconcentrations of 200 ,ug/ml or more. The Serratiastrains gave a pattern of antibiotic susceptibilitysimilar to that of the Enterobacter strains, withalmost all the strains resistant to the cepha-losporins.The terms "resistant" and "sensitive" as used

here are interpretations of in vitro tests. Noimplication is made as to the actual clinicalresponse of these organisms to the antibioticstested. A "sensitive" organism was arbitrarilydefined as one inhibited by any one of the anti-biotics at a concentration of 20 jig/ml or less.An average of 90% of the Klebsiella strains weresensitive to the cephalosporins, compared toonly 15% of the Ezterobacter strains (Fig. 1).Ampicillin appeared to be slightly more effec-tive against Enterobacter strains than againstKlebsiella strains.The sensitivity of the Klebsiella and Entero-

bacter strains to the cephalosporin antibioticswas fairly uniform. If a strain was inhibited byone of these antibiotics, it was usually inhibitedby the other two as well. However, a few strainswere inhibited by one of the cephalcsporinsbut not the other two, or by two but not by thethird. At the 20 Ag/ml level, only 22 strains ex-hibited this peculiarity (Table 4). Consequently,there was a 90% chance that, if an organism wassensitive to one of the cephalosporins, it wouldbe sensitive to the other two as well.To avoid bias that may have been introduced

by our interpretation of the results of the severalbiochemical tests, the atypical strains were ex-

17%AMPICILLIN 3[27%

KLEBSIELLA

ENTEROBACTER

TABLE 4. Klebsiella, Enterobacter, and Serratiastrains sensitivea to only one or two cephalosporin

antibiotics

No. of strains

AntibioticKlebsiela oEnater Serratia

Cephalexin .............. 5 1 0Cephaloridine ......... 3 1 0Cephalothin ............. 0 1 0Cephalothin + cepha-

loridine ........... 2 0 0Cephalothin + cepha-

lexin ............ 6 0 0Cephaloridine + cepha-

lexin ............ 2 1 0

a By definition, "sensitive" means inhibited ata concentration of 20,ug/ml or less.

amined with particular emphasis on their anti-biotic sensitivity patterns. The occasional atypicalEnterobacter strain that was nonmotile or wasornithine-decarboxylase-negative exhibited aboutthe same susceptibility to the cephalosporins asdid the motile, ornithine-decarboxylase-positivestrains. Similarly, the indole-positive or citrate-negative strains of Enterobacter or Klebsiellawere no more susceptible to ampicillin than werethe typical strains. This comparison suggestedthat our original classification of the atypicalstrains was correct and that the pattern of sensi-tivity to selected antibiotics might be a usefuladditional aid in identification of hard-to-classifymembers of the Klebsiella-Enterobacter-Serratiagroup.

DISCUSSIONSERRATIA

CEPHALOTH IN

CEPHALORI DINE

CEPHALEXIN

FIG. 1. Inhibition of Klebsiella, Enterobacter, andSerratia strains by antibiotics at 20 jg/ml or less.Shown as percentages of total tested.

By using several easily performed biochemicaltests (indole production, citrate utilization,urease production, and reactions on triple sugar-iron-agar and motility agar), 88% of our isolatescould be readily identified as Klebsiella, Entero-bacter, or Serratia species. These same tests pro-vide the additional benefit of allowing recogni-tion of other organisms such as E. coli, Citro-bacter, Salmonella, Shigella, Providencia, andProteus. In our study, further biochemical test-ing for classification was needed for only 42strains.When determined in a semisolid medium and

not confused with growth down the side of thetube (6), motility is an adequate procedure fordifferentiating Klebsiella from Enterobacter orSerratia. Twelve per cent of our Enterobacterstrains were nonmotile. The ornithine-decar-boxylase test was slightly more reliable in that

12%

16%

15%

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only 8% of the Enterobacter strains were nega-tive. Routine use of both tests would eliminateerrors in classification.Although identification to the level of species

could be achieved in the majority of instances bythe tests used or by addition of one or two othertests, this information is not included since itwas our purpose to indicate the ease of identifi-cation of the genera and to test antibiotic sus-ceptibility differences inherent in these genera tothe selected antibiotics. Speciation or serologicaltyping would, of course, add to the epidemi-ological value of the information obtained, butsuch an undertaking was beyond the intent of thestudy.The frequency of isolation and the antibiotic

sensitivity of members of the tribe Klebsielleaefrom clinical material vary among reportedseries (2, 4, 6, 8, 16, 20). Klebisella isolates out-number Enterobacter isolates. Lerner and Wein-stein (20) reported a ratio of 2.5:1, Benner et al.(2) reported 4:1, and Eickhoff et al. (8) reported5:1; in our material the ratio was 2.5:1. Almostas variable is the reported antibiotic sensitivityof the different strains-for example, the per-centage of strains of Klebsiella sensitive tocephalothin varies from 22 to 100% (2, 8).The disparities in isolation rate and antibiotic

sensitivity of Klebsiella and Enterobacter strainsare difficult to explain but might be the result ofdifferences in methods used in isolation andidentification or in determination of antibioticsensitivity of the organisms, or they might be areflection of a bona fide difference in geographicdistribution. It has also been suggested thatorganisms isolated from different specimens mayhave different antibiotic susceptibilities, at leastin terms of the sensitivity of Klebsiella to cepha-lothin (16). Consequently, comparisons betweenstudies not specifying the source of isolationmay not be completely valid.Only two of our Serratia strains produced

pigment, and reliance on the production of pig-ment for identification of Serratia is not recom-mended. The combination of motility and pro-duction of ornithine and lysine decarboxylasespermitted the classification of Serratia strainswithout difficulty in most instances, but con-firmation was provided by appropriate fer-mentative reactions in arabinose, rhamnose, andraffinose. Edwards and Ewing (7) reported pig-mentation in only 26% of their strains of Serratia.More recently, Fields et al. (12) described pig-mentation in less than 10% of their clinical iso-lates of Serratia and also pointed out the re-sistance of these organisms to most antibiotics,findings corroborated in our study.

Before the availability of ampicillin there was

little reason to differentiate organisms in the"colon-aerogenes" group because of the simi-larity in antibiotic susceptibility among membersof this large heterogeneous group. Until cephalo-thin was made available, there was little reasonto differentiate between Klebsiella and Entero-bacter species except for epidemiological pur-poses. However, our study suggests that a dis-tinct difference in antibiotic sensitivity does existfor members of this ubiquitous and geneticallyrelated group and that classification into generaby the clinical laboratory can be readily accom-plished, providing useful information to theclinicians.Of the Klebsiella species defined by our sys-

tem, 90% were inhibited by low concentrationsof the cephalosporin antibiotics, compared toonly 15% of the Enterobacter strains. The dif-ferences in sensitivity between the various ceph-alosporins for the Klebsiella were small. Prior re-sults (9) with the agar dilution procedure forantibiotic sensitivity testing indicate the reliabilityof the method, and repeated testing of the sameor more strains would probably further minimizethe differences. Our results most closely parallelthose of Lerner and Weinstein (20), both in thedegree of sensitivity of Klebsiella isolates tocephalothin and in the frequency of isolation ofKlebsiella strains from clinical material. The re-sults of our study contrast sharply with those ofBulger (4) and Eickhoff and et al. (8). By using atube dilution procedure for determining anti-biotic susceptibility, Bulger found that 53% ofthe Klebsiella strains were inhibited by cephalo-thin at 15 Ag/ml or less; Eickhoff et al. reportedthat 22% of Klebsiella strains were inhibited bycephalothin at 25 ,g/ml.

This lack of agreement could be the result ofdifferences in methods of antibiotic sensitivitytesting or in identification of the organisms. Morelikely, it represents one or a combination ofseveral other factors. Benner et al. (2) suggestedthat the adaptive production of cephalosporinaseby some strains of Klebsiella may be responsiblefor differences in antibiotic susceptibility pat-terns. The medical importance of infective drugresistance or resistance transfer factors was re-alized early by the Japanese (23), but until re-cently only the genetic aspects of this phenomenonhave been of interest in this country. As a conse-quence of the use of cephalosporin antibiotics,cephalosporinase production or resistance trans-fer factors may direct the development of resist-ant strains in a hopsital, in isolated areas of ahospital, or in patients under treatment. Thepredominance of such strains would then varyfrom institution to institution. In addition, it is

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well recognized that multiple isolations from thesame patient could influence the statistics.With the wider clinical application of cephalo-

thin and, to a lesser extent, cephaloridine, in-creasing numbers of cephalosporin-resistantKlebsiella may be seen in the future. Althoughthe cephalosporin-resistant strains in this studywere not examined for the presence of resist-ance transfer factors or cephalosporinase ac-tivity, future studies could combine the effortsof the clinical microbiologist, the geneticist, theclinician, and the epidemiologist to evaluate thesignificance of these factors.

Proper bacteriological classification, as ac-complished in the clinical laboratory, providesinformation for a better understanding of thetaxonomic relationship of organisms and forepidemiological studies in tracing sources of in-fection. It can serve also as an aid to the clinicianin the preliminary selection of an antibiotic fortreatment. An understanding of the geneticcapabilities of these organisms will also con-tribute to the taxonomy, epidemiology, andevaluation of antibiotic susceptibility. The actualtreatment of the patient, however, should bebased upon the accurate in vitro determination ofthe antibiotic susceptibility of the organism iso-lated in the clinical laboratory.

LITERATURE CITED

1. Anderson, K. N., R. P. Kennedy, J. J. Plorde, J. A. Shulman,and R. G. Petersdorf. 1964. Effectiveness of ampicillinagainst gram-negative bacteria: In vitro and in vivo studiesof a new antibiotic. J. Amer. Med. Ass. 187:555-561.

2. Benner, E. J., J. S. Micklewait, J. L. Brodie, and W. M. M.Kirby. 1965. Natural and acquired resistance of Klebsiella-Aerobacter to cephalothin and cephaloridine. Proc. Soc.Exp. Biol. Med. 119:536-541.

3. Braun, W. 1946. Dissociation in Brucella abortus: a demon-stration of the role of inherent and environmental factorsin bacterial variation. J. Bacteriol. 51:327-349.

4. Bulger, R. J. 1967. In vitro effectiveness of kanamycin andkanamycin/cephalothin against Klebsiella: comparisonwith other antibiotics. Ann. Intern. Med. 67:523-532.

5. Editorial Board. 1963. Opinion 28, rejection of the bacterialgeneric name Cloaca Castellani and Chalmers and accept-ance of Enterobacter Hormaeche and Edwards as a bac-terial generic name with type species Enterobacter cloacae(Jordan) Hormaeche and Edwards. Int. Bull. Bacteriol.Nomencl. Taxon. 13:38.

6. Edmundson, E. B., and J. P. Sanford. 1967. The Klebsiella-Enterobacter (Aerobacter)-Serratia group: a clinical andbacteriological evaluation. Medicine 46:323-340.

7. Edwards, P. R., and W. H. Ewing. 1962. Identification ofEnterobacteriaceae, 2nd ed. Burgess Publishing Co., Minne-apolis.

8. Eickhoff, T. C., B. W. Steinhauer, and M. Finland. 1966.The Klebsiella-Enterobacter-Serratia division: biochemicaland serologic characteristics and susceptibility to anti-biotics. Ann. Intern. Med. 65:1163-1179.

9. Ericsson, H. 1964. Standardization of methods for conductingmicrobic sensitivity tests: preliminary report of a workinggroup of the International Collaborative Study sponsoredby the World Health Organization, p. 23-25. KarolinskaSjukhuset, Stockholm.

10. Ewing, W. H. 1963. An outline of nomenclature for the fam-ily Enterobacteriaceae. Int. Bull. Bacteriol. Nomencl.Taxon. 13:95-110.

11. Ewing, W. H. 1966. Enterobacteriaceae taxonomy and nomen-clature, December. National Communicable Disease Cen-ter Publication, Atlanta.

12. Fields, B. N., M. M. Uwaydah, L. J. Kunz, and M. N. Swartz.1967. The so-called "paracolon" bacteria: a bacteriologicaland clinical reappraisal. Amer. J. Med. 42:89-106.

13. Fife, M. A., W. H. Ewing, and B. R. Davis. 1965. The bio-chemical reactions of the tribe Klebsielleae, June. NationalCommunicable Disease Center Publication, Atlanta.

14. Finkelstein, R. A., and K. Punyashthiti. 1967. Colonial recog-nition, a "new" approach for rapid diagnostic entericbacteriology. J. Bacteriol. 93:1897-1905.

15. Fleming, P. C., M. Goldner, and D. C. Glass. 1963. Ob-servations on the nature, distribution, and significance ofcephalosporinase. Lancet 1:1399-1401.

16. Herrell, W. E., A. Balows, and J. Becker. 1964. Antibioticsusceptibility studies on the Klebsiella group. Arch. Intern.Med. 114:329-332.

17. Koch, M. L., and H. D. Rose. 1966. Resistance of the Kleb-siella-Aerobacter-Serratia division to cephalothin andampicillin: importance of identification and nomenclature.Amer. J. Clin. Pathol. 46:589-593.

18. Lampe, W. T., II. 1964. Klebsiella pneumonia: a review offorty-five cases and re-evaluation of the incidence and anti-biotic sensitivities. Dis. Chest 46:599-606.

19. Lankford, C. E., and W. Burrows. 1965. Oblique light mi-croscopy as an aid to rapid detection of enteric pathogens,p. 45-50. In Proceedings of the cholera research symposium,Honolulu, 24-29 January 1965. U.S. Government PrintingOffice, Washington, D.C.

20. Lerner, P. I., and L. Weinstein. 1967. The differentiation ofKlebsiella from Aerobacter species by sensitivity to cepha-lothins and penicillins. Amer. J. Med. Sci. 254:63-68.

21. Steers, E., E. L. Foltz, and B. S. Graves. 1959. An inoculareplicating apparatus for routine testing of bacterial sus-ceptibility to antibiotics. Antibiot. Chemother. 9:307-309.

22. Thaler, M. M. 1962. Klebsiella-Aerobacter pneumonia ininfants: a review of the literature and report of a case.Pediatrics 30:206-220.

23. Watanabe, T. 1963. Infective heredity of multiple drug re-sistance in bacteria. Bacteriol. Rev. 27:87-115.

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