In terp a tie n t m icrobiological cross-contam ination a fter d en tal radio graph ic exam in ation
Stuart C. White, DDS, PhD Sebastian Glaze, DDS, Los Angeles Routine intraoral radiography can result in interpa tient microbiological cross-contamination. Oral pathogens can be transferred from a dental patient to the radiographic equipment where they can sur vive for at least 48 hours and contaminate the next patients.
In recent years, several authors have stressed the need for aseptic procedures for instrum ents used in dental p ractice.1 4 T here has been little atten tion, how ever, to the possibility o f cross contam ination in patients from radiographic equipm ent. D uring routine intraoral radiography, viable m icroorganism s can be transferred from the p atien t’s m outh to the radiographic equipm ent by the X -ray technician and be transm itted to o ther patients. T his could require special precau tions in the treatm ent o f patients with various transm issible diseases. T he purposes of this prelim inary study w ere to determ ine w hether there is m icrobiological cross-contam ination o f patients after routine in traoral radiography and to determ ine the viability o f selected oral m icroorganism s on radiographic equipm ent.
Methods and materials
■ E xperim ental design: N ew patients at the U C L A School o f D entistry w ere random ly paired on arrival at the oral radiology clinic. E ach patient received successive full-mouth radiographic exam inations by the sam e technician using the sam e facilities. T he specim ens of oral flora w ere taken before radiographic exam ination o f the first patient and both before and after radiographic exam ination o f the second patient. T he only p a tient pairs considered in this study w ere those in w hich the first patient in each pair was found to have and the second patient before irradiation was JADA, Vol. 96, May 1978 ■ 801
found not to have one of the selected organisms. The presence o f selected organisms in the oral flora o f the second patient after radiographic examination that were not seen immediately be fore radiographic examination, and which were observed in the first patient, indicated interpatient microbiological cross-contamination. A control group consisted o f 25 patients who were tested for the same organisms before and after intraoral radiographic examination. ■ Oral sw abbing procedures: Sterile cotton swabs were used to obtain samples o f the patient’s oral flora. The buccal, lingual, and occlusal sur faces o f the patient’s dentition were swabbed, primarily in the posterior region. In addition, the cylinder and the tube casing on the X-ray machine and the hands o f the technician were swabbed. The swabs were used to inoculate blood agar plates. The X-ray equipment was disinfected with 2% Amphyl* between each pair of patients but not between patients in each pair. In the control group, the X-ray equipment was disinfected after each patient. The technicians washed their hands with p H isoH ext between treatment o f each pa tient. ■ Identification o f organism s: All swabs were used to inoculate sheep blood agar plates, which were then streaked with a sterile inoculating nee dle in an aseptic environment. Cultures were in cubated for 24 hours at 37 C. The colonies were isolated, gram stained (Hucker’s modification), and identified on the basis o f colonial morphol ogy, color, catalase activity, antibiotic sensitiv ity, bile solubility, coagulase synthesis, car bohydrate fermentation, and motility.5'9 In all cases, Streptococcus pyogenes, Staphylococcus aureus, and D iplococcus pneum oniae were selected for study. These organisms were selected because they are known opportunistic pathogens and because there are precise tecnniques for their identification.1011 The cultures were grown in triplicate, and the organisms were considered to be present when one or more col onies were detected on two or three o f the plates.
Results The microbiological oral floras o f 30 pairs of pa tients undergoing radiographic examination were evaluated. The distribution o f the selected or ganisms in the flora of these pairs o f patients is shown in Table 1. These pairs o f patients have been grouped according to the organisms found in the floras of the first patient. In eight pairs of patients, 5 aureus was found in the flora of the first patient but was not found in the flora of the second patient until after radiographic examina tion. 5 pyogenes also appeared to have been transferred among some o f these patients. In the next group of 12 patients, 5 pyogenes was found in the flora of the first patient, but was not found in the flora o f the second patient until after radiographic examination. In the next group o f three patients, D pneum oniae appeared to have been similarly transferred from the first patient to the second in the pair. In the final group o f seven pairs o f patients, organisms were detected in the first patient but none were seen in the second patient. Thus, in these patients, no evidence o f transfer was obtained. To summarize, in 23 of 30 pairs of patients (77%), we obtained evidence of interpa tient microbiological cross-contamination. In these 23 pairs o f patients, there were 26 examples o f interpatient microbiological cross-contamination (two examples each in pairs 3, 4, and 8). There were only two cases (pairs 11 and 20) where organisms were detected in the second patient
THE AUTHORS
WHITE
■ Grow th o f selected organism s: Pure colonies o f S pyogenes, S aureus, and D pneum oniae were identified and placed onto the surface o f a plastic cylinder used on our X-ray machines. The cylin ders were subsequently swabbed, and the extent o f growth determined by culturing for 24 hours at 37 C on a sheep blood agar plate. 802 ■ JADA, Vol. 96, May 1978
GLAZE
Dr. W hite is an associate professor and cha ir man o f the section of oral radiology, UCLA S chool o f Dentistry, Los Angeles, 90024. Dr. Glaze Is a lieutenant in the US Navy Dental Corps, active reserve, and is stationed in the P hilippine Islands. Address requests fo r reprints to Dr. W hite.
T able 1 ■ Distribution of selected organisms in the oral floras of 30 pairs of patients undergoing radiographic examination. First patient__________________ Second patient_________________ Preradiographic culture Postradiographic culture Staph Strep Diplo_____ Staph Strep Diplo_____ Staph Strep Diplo
Pair no. 1 2 3 4 5 6 7 8
X X X X X X X X
O O X X O 0 X X
O 0 0 O O O O O
O O O O O O 0 0
X X 0 0 0 0 X 0
0 0 0 0 0 0 0 X
X X X X X X X X
0 X X X 0 o X X
0 0 0 0 0 0 0 0
9 10 11 12 13 14 15 16 17 18 19 20
0 0 O O X O X O O O O O
X X X X X X X X X X X X
O X O O O 0 0 O O O O X
X 0 X X X 0 X 0 0 0 X 0
0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 o 0 0 0 0
X 0 0 0 X 0 0 0 0 0 0 X
X X X X X X X X X X X X
0 0 X 0 0 0 0 o o o o o
21 22 23
O O O
X O O
X X
0 0 0
o 0 0
0 0 0
0 0 0
X X
X
0 0 0
O X O O O O O
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0
X O O
24 25 26
27
X
28 29
O X X
30
X O X X X O O
after radiographic exam ination or in the first patient o f th at pair. T h e distribution o f the selected organism s in patients in the control group is show n in T able 2. It is apparent th at, in the 30 patients, the p roce dures involved in intraoral radiography did not change the distribution of the organism s detected in the oral flora o f these patients. C ultures disclosed that there w ere m icroor ganism s presen t on the X -ray equipm ent, but not on the hands o f the X -ray technicians after their hands w ere w ashed. It w as assum ed that the cot ton sw abs w ere sterile w hen no grow th w as ob served after they w ere cultured in nutrient broth at 37 C for 24 hours. T h e viability o f the tracer organism s placed on the X -ray equipm ent is show n in Table 3. Each o f these organism s survived under these conditions for at least 48 hours. A t 72 hours after inoculation, only S aureus could be detected. Table 2a Distribution of selected organisms in the oral flora of control patients undergoing radiographic examination. Postirradiation Pre-irradiation No. of patients Staph Strep Diplo Staph Strep Diplo X X 0 0 5 0 0 0 X 0 16 X 0 0 2 2
0 0
0 X
X 0
0 0
0 0
X 0
0 0 0 0 0 0 0
0 0 0 0 0 0
o
o
X 0 0 0 0 0 0 0
Table 3 ■ Relative growth of selected organisms on Xray machines. Time of swabbing after last exposure (hours) Organisms 8 24 48 72 Diplococcus pneumoniae + + + + + + ++ Staphylococcus aureus + + + + + + ++ + Streptococcus pyogenes_________ + + + + + + + + Relative of scale of numbers of organisms cultured: ( - ) no growth; (+)very little growth; (++) moderate growth; (+ ++) heavy growth.
Discussion T h e data described indicate th at interpatient mi crobiological cross-contam ination m ay occur fre quently during routine intraoral dental radiographic exam ination. M ost likely the X -ray equipm ent becom es contam inated w ith the pa tien t’s oral flora w hen the patient is being po sitioned for each intraoral projection by the tech nician, w ho handles the intraoral films after each exposure. A lthough washing will rem ove the o r ganism s from the technician’s hands, the organ isms rem ain viable on the X -ray equipm ent for at least 48 hours if the equipm ent is not disinfected. T he data presented in T able 2 indicate th at there is no significant contam ination o f patients by o r ganism s from external sources during the radiographic procedure. T his is supported by the data
White—Glaze: INTERPATIENT MICROBIOLOGICAL CROSS-CONTAMINATION ■ 803
in T able 1, w hich also show s th at am ong the 23 p a tients showing 26 exam ples o f cross-contam ination, there w ere only tw o cases o f organism s d e tected only in the second patient after radiograph ic exam ination. In this type o f research, the known difficulties associated with consistent sampling th e oral flora add support to the co n clusion th at large num bers o f organism s are tran s ferred from patient to patient. T he dem onstration o f transm ission o f these o r ganism s via the route described does not neces sarily im ply the transm ission o f disease. Indeed, one o r m ore o f our selected organism s, all oppor tunistic pathogens, w ere found in about half o f our putatively healthy test subjects. Induction o f dis ease would result from a com plex interaction b e tw een the transm itted organism s and the host. Successful prediction o f incidence o f transm is sion o f disease would require knowledge o f num bers o f organism s transm itted, as well as m any o ther factors. T herefore, radiographic equipm ent m ust be considered as one o f the sources o f mi crobial contam ination o f the dental patient. A lthough disinfection o f X -ray equipm ent after every radiographic exam ination is probably not routinely indicated, th ere may be specific in stances w hen it would be prudent to carefully disinfect radiographic equipm ent before exam ina tion o f o th er patients. B ecause the radiographic equipm ent is large, wiping agents such as iso propyl alcohol could be used for disinfecting O ther large pieces o f equipm ent in the dental of fice might also becom e contam inated and require disinfection.
Summary Pairs o f patients w ere evaluated for m icrobio logical cross-contam ination after radiographic exam ination. In 30 o f these pairs o f patients there
804 ■ JADA, Vol. 96, May 1978
w as the possibility o f transference o f S pyogenes, S aureus, o r D pneum oniae. Such transference w as observed in 23 (77%) o f these 30 pairs of patients. T he vectors for such transfer include the hands o f the X -ray technician and the radiographic equipm ent. F urther, it was found that each of these organism s w ould survive for at least 48 hours after being placed on an X -ray tube. Since interpatient m icrobiological cross-contam ination can occu r after routine radiographic ex am ination, in som e cases, disinfection o f the radiographic equipm ent is indicated. This investigation was supported in part by NIH General Re search Support Grant no. RR05304. This paper was presented in part at the 52nd general session of the International Association for Dental Research, March 1974, in Augusta, Ga. •National Laboratories, 225 Summit Ave, Montvale, NJ 07645. fWinthrop Laboratories, 90 Park Ave, New York, NY 10016. 1. Walsh, R.F., and Ames, M.l. Reinforcing the aseptic chain in hospital dental practice. J Hosp Dent Prac 6:57 April 1972. 2. MacFarlane, T.W. The dentist and the prevention of serum hepatitis. Br Dent J 132:487 June 1972. 3. Neugeboren, N., and others. Control of cross-contamination. JADA 85:123 July 1972. 4. Ehrlich, A. Clinical control of pathogenic organisms. Dent Asst 41:17 Sept 1972. 5. Bailey, W.R., and Scott, E.G. Diagnostic microbiology, St. Louis, C. V. Mosby Co., 1974, p 20. 6. Pelczar, M.J., Jr. Society of American Bacteriologists. Manual of microbiological methods. New York, McGraw Hill Book Co., 1957, p 16. 7. Difco manual of dehydrated culture media and reagents for microbiological and clinical laboratory procedures, ed 9. Detroit, Difco Laboratories, 1953, p 103. 8. Pickett, M.J., and Manclark, C.R. Laboratory manual for medi cal bacteriology, ed 4. Minneapolis, Burgess Publishing Co., 1966, p 41. 9. Cowan, S.T., and Steel, K.J. Manual for the identification of medical bacteria. New York, Cambridge University Press, 1974. 10. Bailey, W., and Scott, E. Diagnostic microbiology, St. Louis, C. V. Mosby, 1974, chap 16, 17. 11. Lennette, E.; Spaulding, E.; and Truant, J. Manual of clinical microbiology. Washington, DC, American Society for Microbiol ogy, 1974.
Stuart C. White, DDS, PhD Sebastian Glaze, DDS, Los Angeles Routine intraoral radiography can result in interpa tient microbiological cross-contamination. Oral pathogens can be transferred from a dental patient to the radiographic equipment where they can sur vive for at least 48 hours and contaminate the next patients.
In recent years, several authors have stressed the need for aseptic procedures for instrum ents used in dental p ractice.1 4 T here has been little atten tion, how ever, to the possibility o f cross contam ination in patients from radiographic equipm ent. D uring routine intraoral radiography, viable m icroorganism s can be transferred from the p atien t’s m outh to the radiographic equipm ent by the X -ray technician and be transm itted to o ther patients. T his could require special precau tions in the treatm ent o f patients with various transm issible diseases. T he purposes of this prelim inary study w ere to determ ine w hether there is m icrobiological cross-contam ination o f patients after routine in traoral radiography and to determ ine the viability o f selected oral m icroorganism s on radiographic equipm ent.
Methods and materials
■ E xperim ental design: N ew patients at the U C L A School o f D entistry w ere random ly paired on arrival at the oral radiology clinic. E ach patient received successive full-mouth radiographic exam inations by the sam e technician using the sam e facilities. T he specim ens of oral flora w ere taken before radiographic exam ination o f the first patient and both before and after radiographic exam ination o f the second patient. T he only p a tient pairs considered in this study w ere those in w hich the first patient in each pair was found to have and the second patient before irradiation was JADA, Vol. 96, May 1978 ■ 801
found not to have one of the selected organisms. The presence o f selected organisms in the oral flora o f the second patient after radiographic examination that were not seen immediately be fore radiographic examination, and which were observed in the first patient, indicated interpatient microbiological cross-contamination. A control group consisted o f 25 patients who were tested for the same organisms before and after intraoral radiographic examination. ■ Oral sw abbing procedures: Sterile cotton swabs were used to obtain samples o f the patient’s oral flora. The buccal, lingual, and occlusal sur faces o f the patient’s dentition were swabbed, primarily in the posterior region. In addition, the cylinder and the tube casing on the X-ray machine and the hands o f the technician were swabbed. The swabs were used to inoculate blood agar plates. The X-ray equipment was disinfected with 2% Amphyl* between each pair of patients but not between patients in each pair. In the control group, the X-ray equipment was disinfected after each patient. The technicians washed their hands with p H isoH ext between treatment o f each pa tient. ■ Identification o f organism s: All swabs were used to inoculate sheep blood agar plates, which were then streaked with a sterile inoculating nee dle in an aseptic environment. Cultures were in cubated for 24 hours at 37 C. The colonies were isolated, gram stained (Hucker’s modification), and identified on the basis o f colonial morphol ogy, color, catalase activity, antibiotic sensitiv ity, bile solubility, coagulase synthesis, car bohydrate fermentation, and motility.5'9 In all cases, Streptococcus pyogenes, Staphylococcus aureus, and D iplococcus pneum oniae were selected for study. These organisms were selected because they are known opportunistic pathogens and because there are precise tecnniques for their identification.1011 The cultures were grown in triplicate, and the organisms were considered to be present when one or more col onies were detected on two or three o f the plates.
Results The microbiological oral floras o f 30 pairs of pa tients undergoing radiographic examination were evaluated. The distribution o f the selected or ganisms in the flora of these pairs o f patients is shown in Table 1. These pairs o f patients have been grouped according to the organisms found in the floras of the first patient. In eight pairs of patients, 5 aureus was found in the flora of the first patient but was not found in the flora of the second patient until after radiographic examina tion. 5 pyogenes also appeared to have been transferred among some o f these patients. In the next group of 12 patients, 5 pyogenes was found in the flora of the first patient, but was not found in the flora o f the second patient until after radiographic examination. In the next group o f three patients, D pneum oniae appeared to have been similarly transferred from the first patient to the second in the pair. In the final group o f seven pairs o f patients, organisms were detected in the first patient but none were seen in the second patient. Thus, in these patients, no evidence o f transfer was obtained. To summarize, in 23 of 30 pairs of patients (77%), we obtained evidence of interpa tient microbiological cross-contamination. In these 23 pairs o f patients, there were 26 examples o f interpatient microbiological cross-contamination (two examples each in pairs 3, 4, and 8). There were only two cases (pairs 11 and 20) where organisms were detected in the second patient
THE AUTHORS
WHITE
■ Grow th o f selected organism s: Pure colonies o f S pyogenes, S aureus, and D pneum oniae were identified and placed onto the surface o f a plastic cylinder used on our X-ray machines. The cylin ders were subsequently swabbed, and the extent o f growth determined by culturing for 24 hours at 37 C on a sheep blood agar plate. 802 ■ JADA, Vol. 96, May 1978
GLAZE
Dr. W hite is an associate professor and cha ir man o f the section of oral radiology, UCLA S chool o f Dentistry, Los Angeles, 90024. Dr. Glaze Is a lieutenant in the US Navy Dental Corps, active reserve, and is stationed in the P hilippine Islands. Address requests fo r reprints to Dr. W hite.
T able 1 ■ Distribution of selected organisms in the oral floras of 30 pairs of patients undergoing radiographic examination. First patient__________________ Second patient_________________ Preradiographic culture Postradiographic culture Staph Strep Diplo_____ Staph Strep Diplo_____ Staph Strep Diplo
Pair no. 1 2 3 4 5 6 7 8
X X X X X X X X
O O X X O 0 X X
O 0 0 O O O O O
O O O O O O 0 0
X X 0 0 0 0 X 0
0 0 0 0 0 0 0 X
X X X X X X X X
0 X X X 0 o X X
0 0 0 0 0 0 0 0
9 10 11 12 13 14 15 16 17 18 19 20
0 0 O O X O X O O O O O
X X X X X X X X X X X X
O X O O O 0 0 O O O O X
X 0 X X X 0 X 0 0 0 X 0
0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 o 0 0 0 0
X 0 0 0 X 0 0 0 0 0 0 X
X X X X X X X X X X X X
0 0 X 0 0 0 0 o o o o o
21 22 23
O O O
X O O
X X
0 0 0
o 0 0
0 0 0
0 0 0
X X
X
0 0 0
O X O O O O O
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0
X O O
24 25 26
27
X
28 29
O X X
30
X O X X X O O
after radiographic exam ination or in the first patient o f th at pair. T h e distribution o f the selected organism s in patients in the control group is show n in T able 2. It is apparent th at, in the 30 patients, the p roce dures involved in intraoral radiography did not change the distribution of the organism s detected in the oral flora o f these patients. C ultures disclosed that there w ere m icroor ganism s presen t on the X -ray equipm ent, but not on the hands o f the X -ray technicians after their hands w ere w ashed. It w as assum ed that the cot ton sw abs w ere sterile w hen no grow th w as ob served after they w ere cultured in nutrient broth at 37 C for 24 hours. T h e viability o f the tracer organism s placed on the X -ray equipm ent is show n in Table 3. Each o f these organism s survived under these conditions for at least 48 hours. A t 72 hours after inoculation, only S aureus could be detected. Table 2a Distribution of selected organisms in the oral flora of control patients undergoing radiographic examination. Postirradiation Pre-irradiation No. of patients Staph Strep Diplo Staph Strep Diplo X X 0 0 5 0 0 0 X 0 16 X 0 0 2 2
0 0
0 X
X 0
0 0
0 0
X 0
0 0 0 0 0 0 0
0 0 0 0 0 0
o
o
X 0 0 0 0 0 0 0
Table 3 ■ Relative growth of selected organisms on Xray machines. Time of swabbing after last exposure (hours) Organisms 8 24 48 72 Diplococcus pneumoniae + + + + + + ++ Staphylococcus aureus + + + + + + ++ + Streptococcus pyogenes_________ + + + + + + + + Relative of scale of numbers of organisms cultured: ( - ) no growth; (+)very little growth; (++) moderate growth; (+ ++) heavy growth.
Discussion T h e data described indicate th at interpatient mi crobiological cross-contam ination m ay occur fre quently during routine intraoral dental radiographic exam ination. M ost likely the X -ray equipm ent becom es contam inated w ith the pa tien t’s oral flora w hen the patient is being po sitioned for each intraoral projection by the tech nician, w ho handles the intraoral films after each exposure. A lthough washing will rem ove the o r ganism s from the technician’s hands, the organ isms rem ain viable on the X -ray equipm ent for at least 48 hours if the equipm ent is not disinfected. T he data presented in T able 2 indicate th at there is no significant contam ination o f patients by o r ganism s from external sources during the radiographic procedure. T his is supported by the data
White—Glaze: INTERPATIENT MICROBIOLOGICAL CROSS-CONTAMINATION ■ 803
in T able 1, w hich also show s th at am ong the 23 p a tients showing 26 exam ples o f cross-contam ination, there w ere only tw o cases o f organism s d e tected only in the second patient after radiograph ic exam ination. In this type o f research, the known difficulties associated with consistent sampling th e oral flora add support to the co n clusion th at large num bers o f organism s are tran s ferred from patient to patient. T he dem onstration o f transm ission o f these o r ganism s via the route described does not neces sarily im ply the transm ission o f disease. Indeed, one o r m ore o f our selected organism s, all oppor tunistic pathogens, w ere found in about half o f our putatively healthy test subjects. Induction o f dis ease would result from a com plex interaction b e tw een the transm itted organism s and the host. Successful prediction o f incidence o f transm is sion o f disease would require knowledge o f num bers o f organism s transm itted, as well as m any o ther factors. T herefore, radiographic equipm ent m ust be considered as one o f the sources o f mi crobial contam ination o f the dental patient. A lthough disinfection o f X -ray equipm ent after every radiographic exam ination is probably not routinely indicated, th ere may be specific in stances w hen it would be prudent to carefully disinfect radiographic equipm ent before exam ina tion o f o th er patients. B ecause the radiographic equipm ent is large, wiping agents such as iso propyl alcohol could be used for disinfecting O ther large pieces o f equipm ent in the dental of fice might also becom e contam inated and require disinfection.
Summary Pairs o f patients w ere evaluated for m icrobio logical cross-contam ination after radiographic exam ination. In 30 o f these pairs o f patients there
804 ■ JADA, Vol. 96, May 1978
w as the possibility o f transference o f S pyogenes, S aureus, o r D pneum oniae. Such transference w as observed in 23 (77%) o f these 30 pairs of patients. T he vectors for such transfer include the hands o f the X -ray technician and the radiographic equipm ent. F urther, it was found that each of these organism s w ould survive for at least 48 hours after being placed on an X -ray tube. Since interpatient m icrobiological cross-contam ination can occu r after routine radiographic ex am ination, in som e cases, disinfection o f the radiographic equipm ent is indicated. This investigation was supported in part by NIH General Re search Support Grant no. RR05304. This paper was presented in part at the 52nd general session of the International Association for Dental Research, March 1974, in Augusta, Ga. •National Laboratories, 225 Summit Ave, Montvale, NJ 07645. fWinthrop Laboratories, 90 Park Ave, New York, NY 10016. 1. Walsh, R.F., and Ames, M.l. Reinforcing the aseptic chain in hospital dental practice. J Hosp Dent Prac 6:57 April 1972. 2. MacFarlane, T.W. The dentist and the prevention of serum hepatitis. Br Dent J 132:487 June 1972. 3. Neugeboren, N., and others. Control of cross-contamination. JADA 85:123 July 1972. 4. Ehrlich, A. Clinical control of pathogenic organisms. Dent Asst 41:17 Sept 1972. 5. Bailey, W.R., and Scott, E.G. Diagnostic microbiology, St. Louis, C. V. Mosby Co., 1974, p 20. 6. Pelczar, M.J., Jr. Society of American Bacteriologists. Manual of microbiological methods. New York, McGraw Hill Book Co., 1957, p 16. 7. Difco manual of dehydrated culture media and reagents for microbiological and clinical laboratory procedures, ed 9. Detroit, Difco Laboratories, 1953, p 103. 8. Pickett, M.J., and Manclark, C.R. Laboratory manual for medi cal bacteriology, ed 4. Minneapolis, Burgess Publishing Co., 1966, p 41. 9. Cowan, S.T., and Steel, K.J. Manual for the identification of medical bacteria. New York, Cambridge University Press, 1974. 10. Bailey, W., and Scott, E. Diagnostic microbiology, St. Louis, C. V. Mosby, 1974, chap 16, 17. 11. Lennette, E.; Spaulding, E.; and Truant, J. Manual of clinical microbiology. Washington, DC, American Society for Microbiol ogy, 1974.
