JOURNAL OF CLINICAL MICROBIOLOGY, Oct. 1979, p. 425-432 0095- 1137/79/10-0425/08$02.00/0

Vol. 10, No. 4

Compatible Results Obtained from Biotyping and Serotyping in Serratia marcescens P. A. D. GRIMONT,' F. GRIMONT,' S. LE MINOR,' B. DAVIS,2 AND F. PIGACHE' Service des Enterobacteries, Institut Pasteur, F- 75724 Paris Cedex 15, France,' and Enteric Section, for Disease Control, Atlanta, Georgia 303332

Center

Received for publication 22 June 1979

The correspondence between complete serotype and biotype (P. A. D. Grimont and F. Grimont, J. Clin. Microbiol. 8:73-83, 1978) of 474 Serratia marcescens strains was studied. Of 127 serotypes, 70 were represented by two or more strains. For 60% of these serotypes, strains of the same serotype belonged to one biotype. However, for 91% of serotypes, strains of the same serotype belonged to one biogroup-i.e., a group of closely related biotypes. Biogroups are Al (Ala, Alb); A2/6 (A2a, A2b, A6a, A6b); A3 (A3a, A3b, A3c, A3d); A4 (A4a, A4b); A5/8 (A5, A8a, A8b, A8c); and TCT (TCT, TT). Only two serotypes were composed of a mixture of pigmented and nonpigmented biogroups. Pigmented biogroups (Al and A2/6) were otherwise differentiated from nonpigmented biogroups (A3, A4, A5/8, and TCT) by serotyping. Some biogroups preferentially occurred in some O serogroups: A4 in 01; A2/6 in 06, 08, and 014; and A3 in 09, 012, and 015. Three H serogroups were found to be biochemically homogeneous: Hi, H7, and H20 were respectively and uniquely composed of biogroups A4, TCT, and A3. A square matrix of 0 versus H serogroups, with the corresponding biogroup for each O x H combination, was used for comparisons between 0 groups and between H groups. Identical patterns of biogroups were shown by serogroups 06, 08, and 014. Taxonomical, ecological, and practical consequences of these findings are discussed.

Since the late 1950's, the number of reports documenting human infection by Serratia marcescens has increased dramatically (1, 6, 7, 15, 20). The increased interaction between humans and Serratia is mostly seen in hospitalized human patients with modified resistance and flora. The causative agents are usually nonpigmented S. marcescens biotypes that are resistant to one or more antibiotics. The reasons for the emergence of S. marcescens in human pathology are only partially understood. Some of these reasons are associated with patient status and with modem hospital practice. These have been thoroughly studied (19). Other reasons are associated with the bacteria involved and are poorly under-

cescens consists of 20 0 antigens and 20 H antigens (4, 11, 12, 17). These antigens may be further subdivided (16). In many cases, absorbed O and H antisera must be used because extensive cross-reactions often occur (4, 11, 12, 17). In this article we compare (i) biotyping by carbon source utilization tests, tetrathionate reduction, horse blood hemolysis, and pigment production with (ii) typing with absorbed 0 and H antisera. MATERIALS AND METHODS

Bacterial strains. We tried to include all S. marcescens serotypes available at the Center for Disease Control and the Institut Pasteur. Incomplete serotypes, i.e., rough or nonmotile strains, were, however, stood. omitted (except for 016:H- because no motile strain Nonpigmented S. marcescens strains often with 016 is known). Four hundred seventy-five S. differ from pigmented strains by biochemical marcescens strains were studied.- AH1 strains were idencharacters (8, 9), and it was hypothesized that tified as S. marcescens by publirhed procedures (4). by the nonpigmented and pigmented S. marcescens Furthermore, the identification was checked adonitol, L-arabinose, D-sorbitol, were different infra-specific taxonomic entities pattern of growthasonsole carbon sources (9). and L-rhamnose (9). New H antigens were independently Serotyping. Serotyping was first used in the taxonomy of proposed by Traub and Kleber (17) and Le Minor and Serratia in 1906 (10). The systematic inventory Pigache (11). The same new H antigens were, however, of S. marcescens 0 antigen, however, began only sometimes given different H numbers. A new H noin 1957 (3) and that of H antigens began in 1959 menclature has been published by Le Minor and Pi(5). The present serotyping system for S. mar- gache in accord with W. H. Traub (12). To avoid 425

426

GRIMONT ET AL.

J. CLIN. MICROBIOL.

these serotypes (60%) are homogeneous with respect to biotypes. The remaining 28 serotypes correspond to more than one biotype each. The different biotypes that correspond to the same serotype, however, often differ by only one biochemical test, e.g., biotypes Ala and Alb only differ by growth on DL-carnitine, and A2a and A6a differ by growth on quinate (see reference 8). Groups of closely related biotypes (hereinafter called biogroups) can be formed which increase the correspondence with serotyping. These biogroups are Al (Ala, Alb); A2/6 (A2a, A2b, A6a, A6b); A3 (A3a, A3b, A3c, A3d); A4 (A4a, A4b); A5/8 (A5, A8a, A8b, A8c); and TCT (TCT, TT). Of the nine tests reported to define biotypes (8), only five can differentiate biogroups. Biochemical characteristics of these biogroups are summarized in Table 3. Sixty-four (91%) serotypes are homogeneous with respect to biogroups. Only two serotypes (05:H2 and 05:H13) are composed of a mixture of pigmented and nonpigmented biotypes. Pigmented biogroups (Al and A2/6) can be differentiated from nonpigmented biogroups (A3, A4, A5/8, and TCT) by serotyping. Correlation between biogroups and 0 and H serogroups. Table 4 allows a comparison of 0 serogroups (lines) and H serogroups (columns) with biogroups (intersection of lines and columns). All 0 groups are divided into several biogroups. Some biogroups, however, preferentially occur in some 0 group: A4 in 01; A2/6 in 06, 08, and 014; A3 in 09, 012, and 015. Three H groups are biochemically homoRESULTS Biochemical homogeneity of serotypes. geneous: Hl (9 serotypes, 76 strains) is uniquely The correspondence between serotype and bio- composed of A4 strains. H7 (6 serotypes, 28 type of 474 S. marcescens strains of various strains) is composed of 25 biogroup TCT strains strains. H20 (3 serotypes, 11 geographic origin is given in Table 2. One strain and 3 atypical is strains) uniquely composed of biogroup A3 of serotype 015:H8 was auxotrophic, and its biotype could not be determined (not included strains. Table 4 can also be used for comparison in Table 2). Of 127 serotypes listed, 70 are rep- among 0 serogroups and among H serogroups. resented by two or more strains. Forty-two of 06, 08, and 014 display the same patterns of biogroups: for each H antigen, the same biogroup is found in 06, 08, and 014. Similarities TABLE 1. Nomenclature of new H antigens in S. are also observable between 03 and 012 and marcescens between 09 and 015. Among H serogroups, H2 Original designation and H3 groups display the same patterns of New designabiogroups. Some similarities are also observable Traub and KleLe Minor and tion" (12) ber (17)h Pigache (11) between H8, H9, HIO, and Hll. Serotypes composing biogroups. BioH14 H16 H14 groups with the corresponding serotypes are H15 H15 H17 H14 given in Table 5. Biochemically heterogenous H16 H16 H17 H17 serotypes 02:H4, 05:H 13, 013:H4, 013:H 11, and H18 H18 013:H17 are not listed. Serotype 05:H2 is shown H19 H19 in parentheses because it is mainly composed of H15 H20 one biogroup. Table 5 shows that biogroups are Used in this paper. antigenically distinct. Of S. marcescens seroNumbers in parentheses are references. types, 20% correspond to pigmented biogroups

confusion, the correspondence between original and new H nomenclature is given in Table 1. The new nomenclature has been used in this article. Published methods for the preparation of 0 and H antisera were exactly followed at the Center for Disease Control (4) and Institut Pasteur (11, 12). The same reference strains were used for both preparation and control of 01 to 015 antisera and HI to H13 antisera (4). Preparation and control of 016 to 020 antisera, Co antiserum, and H14 to H20 antisera were done as described by Le Minor and Pigache (11, 12). The 0-antigen group was determined by the tube agglutination method with titration. Procedures followed at the Center for Disease Control (4) and Institut Pasteur (12) were detailed elsewhere. Strains were passaged through semisolid agar medium before tube agglutination with H antisera by the procedure of Edwards and Ewing (4) at the Center for Disease Control and Le Minor and Pigache (11) at Institut Pasteur. Furthermore, the H-immobilization test (11) was performed on all strains serotyped at Institut Pasteur. Biotyping. Methods for S. marcescens biotyping were followed exactly as published elsewhere (8). Peptone-glycerol agar (peptone [Difco], 5 g; glycerol, 10 ml; agar [Difco], 20 g; and distilled water, 1 liter) incubated for 14 days at 30°C was used to enhance pigment production (22). By convention, a "pigmented strain" produces the water-insoluble pigment prodigiosin irrespective of how capricious this production is, and a "nonpigmented" strain never produces prodigiosin. Occasional strains that produce a pink, watersoluble pigment that diffuses in the agar leaving the colony colorless will be considered nonpigmented since such diffusible pigment is unrelated to prodigiosin (22, 23).

VOL.

BIOTYPING AND SEROTYPING OF S. MARCESCENS

10, 1979

427

TABLE 2. Correspondence of serotype and biotype in 474 S. marcescens strains

S,Bith Blotype Serotype't

No. of strains

2 6 1

O1:H1 O1:H4 O1:H5 O1:H7

A4a A4a A4a TCT

O1:H8 Ol:H12

A4a A4a A4a

2 1 1 13

02:H6 02:H7 02:H8 02:H13 03:H1

A3d A8b A3c A3d A4a TCT A4a TT A4a

5 2 1 1 1 2 2 1 10

03:H4 03:H5

A4b A4a A3d

1 1 9

03:H9 03:H10 03:H11 03:H12

A3c Ala A3a A5 A8a

1 1

03,6,7e:H2 03,6,7:H4 03,6,7:H13 04:H1

A8c A2a A8a A2a A4a

04:H4 04:H5 04:H7

A4a A5 TCT

04:H10 04:H12 04:H13 05:H1

A4a A5 A4a A4a

3 1 8 1 1 2 1 8

05:H2

Ala

15

Alb

1

A3a Ala

1 1

Alb A3d

1

A4a TCT A4a A4a A4a

2 3 3 1 1

02:H1

02:H4 02:H5

05:H3

05:H4 05:H6 05:H7 05:H8 05:H9 05:H10

Atypicald

Atypicalf

9

Geographic origin

(no. of strains)" USA: Massachusetts (1), Washington (1) USA: Illinois (2 + 1), Minnesota (2), Michigan (1) USA: Georgia USA: Ohio (2), Tennessee (6), Illinois (1). USA: Virginia (2) France: Rh6ne-Alpes USA: New York USA: Illinois (2), Indiana (1), Massachusetts (1), Georgia (1), Minnesota (2), Virginia (1); France: Aquitaine (1), Auvergne (1), Paris (1); India (1); Tunisia (1) USA: Massachusetts (4), Delaware (1) USA: Tennessee (1); Tunisia (1) USA: Florida USA: New Jersey USA: Georgia USA: New York (2) France: Alsace (2) France: Alsace Portugal (2); France: Aquitaine (2 + 1), Normandie (1), Alsace (2), Paris (1); Spain (1) USA: Georgia USA: Illinois USA: Illinois (2), Texas (4), Pennsylvania (1), Florida (1), Connecticut (1); Philippines (1); Collection (1) USA: Indiana USA: Michigan

Spain (3)

3 1 22 2 1 1

1 31

1%,

1

USA: New York France: Aquitaine (3), Paris (1), Languedoc (2), Nord (3), Loire (3), Bretagne (1), Normandie (1), Alsace (1); Tunisia (2) France: Aquitaine (2) USA: Virginia USA: North Carolina Venezuela USA: Indiana (1), Massachusetts (2), California (1), South Carolina (2); France: Paris (3), Centre (1), Aquitaine (11); Portugal (9); NRC 1005: Serratia anolium USA: California (2), Montana (1) USA: Pennsylvania USA: Tennessee (7), Georgia (1) France: Centre USA: Pennsylvania USA: Ohio (1), District of Columbia (1) USA: Massachusetts USA: Minnesota (2), Nebraska (2), Georgia (1); France:

Aquitaine (2), Rh6ne-Alpes (1)

USA: Virginia (2), Illinois (2), Georgia (1); Brazil (2); France: Champagne (1), Rh6ne-Alpes (4 + 1), Paris

(1); Spain (1) Martinique

France: Alsace USA: Colorado USA: Illinois USA: Ohio USA: Minnesota (1), Florida (1) USA: Michigan (1); France: Alsace (2) USA: Georgia (1), Maryland (1), New Hampshire (1) USA: Massachusetts USA: Nebraska

428

GRIMONT ET AL.

J. CLIN. MICROBIOL. TABLE 2-Continued

Blotype Serotype' eBiotype

05:H11 05:H13

Atypicalg Alb

05:H15 05:H[16]h 05:H19 06:H2

A3c A6a A3a A4a TCT A2a

Geographic origin

No. of strains

(no. of strains)'

1 3 1 2 6 1 2 7

USA: Georgia France: Paris (1), Charente (1), Alsace (1) USA: Nevada USA: Massachusetts (2) France: Aquitaine (2), Alsace (2); Tunisia (2) France: Aquitaine France: Alsace (2) USA: Virginia (2), Illinois (2); France: Alsace (2), river

3 1

USA: Minnesota (1), Tennessee (1); France: Alsace (1) France: Aquitaine France: Paris USA: New York USA: North Carolina ATCC 274 France: Aquitaine USA: Massachusetts (2); Puerto Rico (2); France: Paris (1 + 1) France: Paris USA: Illinois (2); France: Paris (1 + 1), Alsace (3), Aquitaine (1) USA: Ohio (1), Illinois (1) USA: North Carolina USA: Georgia USA: Oklahoma USA: Massachusetts USA: Mississippi

(1) A2a A5 A8a A3d A2a A2a A2b A5

1 1 6

A8a A8b

1 8

06:H13 06:H20 07:Hl 07:H4 07:H7 07:H10 08:H2 08:H3 08:H4 08:H8 08:H9 08:H10 08:H12 09:H1 09:H2 09:H4 09:H8 09:H9 09:H11 09:H12 09:H17

A2a A3b A4a TT TT A4a A6a A6a A5 A6b A2a A2a A8a A4a A3b A3d A3a A3a A3a A5 A3c

2

O10:H6 O10:H8 O10:H9 O10:H13

Ala TC

06:H3 06:H4 06:H5 06:H8 06:H10

06:H12

1 1 1

1 1 1 1 1 1 2 1

1 1 1 2 2 1 1

1 1

3 1 10

Uruguay USA: Wisconsin (1), Maryland (1) USA: North Dakota France: Rh6ne-Alpes USA: New York Denmark France: Alsace (2) USA: Pennsylvania (1 + 1) USA: Illinois USA: Massachusetts USA: Pennsylvania USA: Pennsylvania USA: Pennsylvania (1), Ohio (1); Spain (1) USA: Michigan Portugal (1); France: Paris (3 + 1), Alsace (3), Loire (1 + 1)

Oll:H4 O11:H13 012:H1 012:H4 012:H5 012:H9

012:H11 012:H13 012:H16

TQ'

3 2 1

A4a A4a A3a

1 3 1 1 1 6

A3a A3b A3a A3b Alb A3b

2 1 2 1 1 1

Ala TCT

Atypical-

USA: Pennsylvania (2), Puerto Rico (1) USA: Pennsylvania (1), Ohio (1) USA: Ohio USA: Michigan USA: Massachusetts (3) USA: Massachusetts USA: Illinois USA: Illinois USA: Ohio (1), Pennsylvania (1); France: Auvergne (2), Alsace (2) USA: Idaho (1), Minnesota (1) USA: Minnesota USA: Illinois (1), Massachusetts (1) France: Alsace USA: Illinois Switzerland

BIOTYPING AND SEROTYPING OF S. MARCESCENS

VOL. 10, 1979

429

TABLE 2-Continued Serotype'

012:H17 012:H18 012:H20 013:H1 013:H4 013,19k:H4

013:H5 013:H7 013:H8 013:Hll 013:H12 013:H13 013:H17 014:H2 014:H3 014:H4

014:H5 014:H6 014:H8 014:H9 014:H1O 014:H11 014:H12

014:H13 014:H20 015:H3

Biotypeh

A3b A3b A3a A4a A4b A3d TT TCT A2a TCT TT A3a A3a A4a TCT A4b A3a TCT A2a A6a A2a A6a A5

No. of strains

3 1 2 6 1 1 1 1

3 1 1 1 1 1 3 2 4 1 3 3 2 1 18

A8a A8b A3a A3d A3a A2a A6a A2a A2a A6a A6b A2a A5

9 5 1 1

A8a

9

A8b

16

1

2 3 1 2 3 1

1

9

A2a

1

A3a A3b

015:H5 015:H8

A3a A3a A3b

8 1 1 2 2 1

015:H9 015:H11 015:H12 015:H13 015:H new 016:017:H4 018:H[9]' 018:H16

A3b

1

A3c

A3a

1

A5 A8b A3b TT

2 1

A3d TCT

1 1 4

TCT

1

2

Geographic origin (no. of strainsY

France: Alsace (3) France: Alsace France: Alsace (2) USA: Minnesota (2); France: Paris (3), Aquitaine (1) USA: Washington USA: Ohio USA: Massachusetts France: Alsace USA: District of Columbia (1); France: Normandie (2) USA: Pennsylvania USA: Michigan USA: New York USA: Tennessee USA: Pennsylvania France: Aquitaine (3) USA: Washington (2) France: Alsace (4) Germany USA: Minnesota (2); France: Aquitaine (1) USA: Illinois (1), North Carolina (1); France: river (1) USA: Illinois (1); France: Rh6ne-Alpes (1) USA: Texas USA: Texas (1); France: Aquitaine (12), Paris (2), Alsace (2), Loire (1) France: Alsace (6), Aquitaine (1), Centre (1), Lorraine (1) USA: Texas (1 + 1), Minnesota (2), Michigan (1) USA: Ohio USA: Pennsylvania USA: Ohio USA: Illinois (1); France: Alsace (1) USA: Illinois (1), Ohio (2) USA: North Carolina USA: North Carolina France: Alsace (2), Paris (1) France: Alsace France: Rh6ne-Alpes England (1); USA: Massachusetts (1), Kentucky (1), District of Columbia (2); France: Normandie (2), Rh6ne-Alpes (2) USA: Texas (1), Indiana (1), Massachusetts (2); Puerto Rico (2); France: Paris (1), Alsace (1); England (1) Portugal (1); Denmark (2); USA: Illinois (2 + 2), Minnesota (2), Massachusetts (3); France: Aquitaine (3), Auvergne (1) USA: Illinois France: Alsace (7), Paris (1) USA: Ohio France France: Aquitaine (2) USA: Massachusetts (2) USA: Ohio USA: Ohio USA: Massachusetts USA: California (1), Unknown (1) USA: Massachusetts USA: Ohio France: Nord (2) France: Aquitaine France: Alsace France: Aquitaine (3), Rh6ne-Alpes (1)

430

J. CLIN. MICROBIOL.

GRIMONT ET AL. TABLE 2-Continued Serotype"

Biotypeh

No. of

strains

Geographic origin (no. of strains)"

1 France: Alsace TCT 019:H5 France: Alsace (2); USA: Ohio (1), North Carolina (1) 4 TCT O19:H14 3 France: Alsace (3) TC 020:H12 4 France: Alsace (4) A3a "Co"H17 5 Tunisia (5) TQ' New O:H16 O antigens as described by Edwards and Ewing (4) and Le Minor and Pigache (12). H-antigen nomenclature is as given by Le Minor and Pigache (12). Biotype according to Grimont and Grimont (8). States of the United States (USA), regions of France, and other countries. When several strains from a same state (USA) or region (France) are known to come from different hospitals, the figures are indicated separately: e.g., Illinois (2 + 1). " Biotype A5 without growth on 4-hydroxybenzoate. e Strains were agglutinated by 03, 06, and 07 antisera at similar titers. These strains could not saturate these antisera. They were not agglutinated by 06 antiserum absorbed by 07 bacterial cells or by 07 antiserum absorbed by 06 bacterial cells. f Positive tetrathionate reduction. Growth on quinate, 4-hydroxybenzoate, and carnitine. g Nonpigmented A6a strain. h Related to H16. 'Positive tetrathionate reduction, pigmentation, growth on quinate and 4-hydroxybenzoate (differs from biotype A6 by lack of growth on erythritol). ' Tetrathionate reduction negative. Growth on quinate, 4-hydroxybenzoate, and benzoate. kThe strain was agglutinated by 013- and 019-absorbed antisera. 'The strain could not completely saturate H9 antiserum. TABLE 3. Summary of biochemical characteristics of S. marcescens biogroups Growth on:

Tetrathio-

P

Prodg-

iosin Biogpnate Eryth- Ben- Quin- reduc- proBiogroup duction ate ritol zoate tion

Oh +a + + + Al 0 + D' + + A2/6 0 0 0 + + A3 0 0 0 + D A4 0 0 0 + + A5/8 0 0 0 0 + TCT " +, growth in 4 days, or tetrathionate reduced in 2 days,0 or prodigiosin produced (100% strains positive). O, No growth in 14 days, or tetrathionate not reduced in 2 days, or prodigiosin not produced on any medium tested (100% strains negative). D, Different biotypes.

(Al and A2/6); 47% of serotypes correspond to erythritol-positive, nonpigmented biogroups (A3 and A4); and 30% of serotypes correspond to erythritol-negative, nonpigmented biogroups (A5/8 and TCT). DISCUSSION Taxonomical consequences. Biotypes had been formed (8) by subdivision of S. marcescens subgroups obtained by numerical taxonomy (9). The biogroups in this paper resemble the original subgroups (9). Biogroups A2/6 and A5/8, however, result from the fusion of subgroups A2

and A6 and the fusion of subgroups A5 and A8, respectively. Biogroups can be identified by pigmentation and biochemical characteristics and are antigenically distinguishable. Polynucleotide sequence relatedness between S. marcescens biogroups is now under study. Ecological consequences. Our data do not support the general belief (although rarely expressed in written form) that Serratia strains may have lost their pigmentation after passage in hospitalized patients under antimicrobial therapy. The role of pigmented S. marcescens biogroups in human infections probably has not changed since the late 1800's. Nosocomial S. marcescens infections are commonly caused by nonpigmented biogroups (2, 4, 7, 8, 20). The biogroup/serotype correspondence given in Table 2 allows some extrapolation from the literature. In the first paper in which complete serotyping of S. marcescens strains was reported (5), 31 out of 54 (57%) human strains reported belong to serotypes that correspond to biogroup A4, 2 strains (4%) correspond to biogroup A5/8, 1 strain (2%) corresponds to biogroup TCT, 8 strains (15%) correspond to biogroup Al, and 6 strains (1 1%) correspond to biogroup A2/6. The most frequently occurring nosocomial S. marcescens serotypes reported by Wilkowske et al. (21), Wilfert et al. (20), and Maki et al. (13) correspond to biogroups A4 and A5/8. The Nashville epidemic (15) was due to serotype 01: H7, a member of biogroup TCT. On the other hand, the red diaper syndrome (14) was caused

VOL. 10, 1979

BIOTYPING AND SEROTYPING OF S. MARCESCENS TABLE 4. Biogroup associated with S. marcescens 0 and H serogroups

O anti-

H antigen

gn

1 2 3 4 5

1

2 3

'4a 4

4

516

7

8

4

4 3 4

T T

4

19 110

1

11

13

14 1516

17118

41 3/5 4T 4 33 1 3

4 4 5 4 1/3 1 3 62 2 5 3 7 4 fT 8 225 9 4 3 3

10 11 12 13 14 15 17 18 19 20

431

T

4

1

4 4

2

T 4 3 3/T 2 2 5 3 3 3 3

4 4 44 2 ~2 T 4 2 2 2 3 3

TC TQ 3 2 3

3

5 3 5 3

3/4 T 2 3

334

3

1

3

T 3

4 1/2/3 5 2

T

2

2 3

5 5

1920

1 4

3 3

3

3

3/T

23

5

T

T

T

T TC

Co3 i:~~~~. ~~~ --1a otatlin OI~~~s is sunpimfed as tonows: 1 =A-. oiogroups Al, 2 = A2/6, 3 Atypical strains are not shown. b 0 group 16 and complex antigens are deleted froma the table. %

TABLE 5. Significanta serotypes corresponding to each S. marcescens biogroup Biogroups

Serotypes

Al A2/6

(05:H2),b 05:H3, 010:H6 06:H2, 06:H3, 06:H1O, 06:H13, 08:H3, 013:H5, 014:H2, 014:H3, 014:H8, 014:H1O A3 03:H5, 03:Hll, 05:H15, 09:Hll, 09: H17, 012:H5, 012:H9, 012:Hll, 012: H17, 012:H20, (013:H17), 014:H5, 014:H20, 015:H3, 015:H5, 015:H8, Co:H17 A4 01:Hl, 01:H4, 02:H1, 02:H8, 03:H1, 04: HI, 04:H4, 05:H1, 05:H6, 05:H8, 09: Hi, 013:H1, 013:H13 A5/8 03:H12, 04:H12, 06:H4, 06:H12, 08: H12, 014:H4, 014:H12, 015:H12 TCT (01:H7),' 02:H7, (04:H7), 05:H7, 05: H19, 011:H4, 013:H7, 013:H12, 016:-, 019:H14 TCd 010:H8, 020:H12 a Serotypes represented by at least two strains. b Parentheses indicate that exceptions occur. c Exceptional strains that do not belong to biogroup TCT are biochemically atypical. d Rare biotype (8).

by serotype 06:H2, a member of pigmented biogroup A2/6. Farmer et al. (7) distinguished between common source outbreaks and cross-infection outbreaks. Fifty percent of common source outbreaks were caused by pigmented S. marcescens, whereas 89% of cross-infection outbreaks were caused by antibiotic-resistant, non-

A.

=

A3,

4

=

A4, 5

=

A5/8, T

=

TCT.

pigmented S.

marcescens (7). These and other reports (8) suggest that although nonpigmented S. marcescens biogroups are a real threat in hospital practice, pigmented biogroups cause less trouble-if massive amounts are not injected into the patient. The ecology of S. marcescens biogroups in humans and in the natural environment is now under study. Practical consequences. S. marcescens biogroups are subdivided by serotyping, and serotypes are subdivided by biotyping. A combination of both serotyping and biotyping systems should be helpful in epidemiological studies. Serotyping may be difficult when cross-reactions occur. Determination of biogroup may provide a way of checking the serotyping. In the case of discrepancies between the actual serotyping re-

sult and the result that could be expected from Table 2, the antigenic structure of the strain should be investigated further, including crossabsorption of sera with the test strain. Determination of biogroup or biotype may be very useful when the strain to be serotyped is either rough or nonmotile. The 0 serogroups and H serogroups that display similar biogroup patterns in Table 4 happen to give extensive antigenic cross-reactions: 06, 08, and 014; 09 and 015; and to some extent H8, H9, and H10. The validity of the distinction between 06 and 014 has been questioned (12, 18) because it is technically difficult to distinguish between them. Biochemical studies of serotyped strains may

stimulate a revision of S. schema.

marcescens

antigenic

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The reader may wonder why none of the reactions defining biogroups (or biotypes) is a common test in clinical laboratories. Our biotyping system derives from a numerical taxonomic treatment of the results of many tests, including carbon source utilization tests (9). The results showed that carbon source utilization tests were the most discriminant tests for species and biotype identification. No common test was found useful in biotype determination (8, 9). We recommend the determination of at least S. marcescens biogroups as a first step in epidemiological studies of Serratia infections. Isolates belonging to different biogroups are unlikely to be of a same serotype. LITERATURE CITED 1. Altemeier, W. A., W. R. Culbertson, W. D. Fullen, and J. J. McDonough. 1969. Serratia marcescens septicemia. A new threat in surgery. Arch. Surg. 99: 232-238. 2. Clayton, E. D. W., and A. von Graevenitz. 1966. Nonpigmented Serratia marcescens. J. Am. Med. Assoc. 197:1059-1064. 3. Davis, B. R., and J. M. Woodward. 1957. Some relationships of the somatic antigens of a group of Serratia marcescens cultures. Can. J. Microbiol. 3:591-597. 4. Edwards, P. R., and W. H. Ewing. 1972. Identification of Enterobacteriaceae, 3rd ed. Burgess Publishing Co.,

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