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Journal of Clinical Microbiology, April 2000, p. 1628-1631, Vol. 38, No. 4
Kairos Scientific Inc., Santa Clara,
California 95054
Received 8 October 1999/Returned for modification 30 December
1999/Accepted 28 January 2000
rRNA-based molecular phylogenetic techniques were used to identify
the bacterial species present in the ear fluid from a female patient
with otitis externa. We report the identification of
Staphylococcus intermedius from the patient and a possible
route of transmission. Analysis of 16S ribosomal DNA restriction
fragment length polymorphisms indicated that the dominant species
present was S. intermedius. A pet dog owned by the patient
also was tested and found to harbor S. intermedius. In
humans, the disease is rare and considered a zoonosis. Previously,
S. intermedius has been associated with dog bite wounds,
catheter-related injuries, and surgery. This study represents the first
reported case of a noninvasive infection with S. intermedius.
Associated frequently with animals
and rarely found in humans, Staphylococcus intermedius was
first described as a new species in 1976 and differentiated from
Staphylococcus aureus and Staphylococcus epidermidis based on biochemical and microbiological tests
(9). S. intermedius has been isolated from the
skin, hair, and gingiva of normal healthy dogs (2, 21) and
has been found as a microbial inhabitant of several dog bite wounds
(4, 14, 21, 22). Although this bacterium can be pathogenic
in animals, it has been identified infrequently in humans (13, 15,
17, 23) and rarely causes disease. In the few instances of
disease, S. intermedius was isolated only from patients who
had undergone invasive procedures, for example, in a catheter-related
bacteremia case (26) and following a coronary artery bypass
grafting (8). Because S. intermedius is an
uncommon component of the normal human microbiota (17) and
can be associated with animal bite wounds, it represents a true
zoonotic pathogen.
Methods for bacterial identification have been undergoing rapid change
over the past decade, and molecular phylogenetic techniques are rapidly
becoming the procedures of choice (10, 19, 25). Phylogenetic
classification based on the 16S rRNA gene has clarified the taxonomy of
many bacterial groups from genera to families and has led to the
discovery of several new divisions (phyla) (12, 18). PCR
amplification of the 16S rRNA gene directly from a sample of mixed
microbiota alleviates the requirement for culturing (18).
This is important, since most microorganisms (greater than 99% on
earth) have not been cultured, and it may be very difficult to do so
because many of them exist in complex natural communities, such as biofilms.
In the present study, we report the bacterial species found in the ear
fluid of an otherwise healthy female patient with otitis externa, and
we correlate these findings with the possible source of the infection,
an indoor pet dog. S. intermedius was identified as the
major bacterial component of ear fluid from this patient. It was also
found that her indoor pet dog harbored S. intermedius. Following antibiotic treatment and recovery of the patient from otitis
externa, the microbial population in the patient lacked S. intermedius.
Sample collection.
A sterile BBL CultureSwab (Becton Dickinson
Microbiology Systems, Sparks, Md.) was used to collect ear fluid from a
38-year-old female patient with otitis externa, who was otherwise
healthy. Samples from a 2-year-old dog (golden retriever) owned by the patient were collected in the same manner as above. The canine samples
were from the ear, back, and chest. Both human and canine samples were
stored at DNA extraction and PCR.
Genomic DNA was obtained with a Soil
DNA Isolation Kit according to the manufacturer's recommendation (Mo
Bio Laboratories, Solana Beach, Calif.) and gave high-quality DNA
suitable for PCR. The 16S and 23S ribosomal DNA (rDNA) primers used in
this study are listed in Table 1. The
following primer pairs were used for amplification of the genomic DNA
with their specificities listed parenthetically: 515F-1492R
(universal), 27F-1492R (Bacteria domain members only),
StaphF-1492R (Staphylococcus and related genera), and
ITS1F-ITS1R (S. intermedius and closely related species).
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Molecular Phylogenetic Evidence for Noninvasive
Zoonotic Transmission of Staphylococcus intermedius from
a Canine Pet to a Human
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80°C until processed. A control sample, S. intermedius ATCC 51874, was acquired from the American Type Culture Collection (ATCC; Manassas, Va.) and stored at
80°C.
TABLE 1.
Oligodeoxynucleotide primers used for PCR amplification
of bacterial DNA (16S rDNA, ITS, or 23S rDNA) from human, canine,
and control samples
Cloning and RFLP analysis. PCR products were purified by a QIAquick PCR purification kit (Qiagen, Chatsworth, Calif.) and cloned with the TOPO TA cloning kit pCR2.1-TOPO vector (Invitrogen Corporation, Carlsbad, Calif.), according to the manufacturer's recommendations. rDNA inserts from pCR2.1 vector clones were reamplified by PCR with vector primers to the T7 and SP6 promoter sites, approximately equidistant from the 5' and 3' ends of the insert. Restriction fragment length polymorphism (RFLP) analysis was used to estimate the diversity of bacteria in the patient sample. DNA was digested with MspI and HinP1I (New England Biolabs, Beverly, Mass.). The digested DNA was separated on a 3% MetaPhor gel (FMC Bioproducts, Rockland, Maine) in 1× Tris-acetate-EDTA for about 2 h at 50 V.
Sequencing and phylogenetic analysis. PCR products were sequenced on an ABI Prism 377XL automated DNA sequencer. 16S rDNA sequences were compared to known sequences in GenBank (5) with the advanced gapped BLAST (basic local alignment search tool) algorithm (3). The sequences were compiled in Chromas version 1.3 (Conor McCarthy, Griffith University, Brisbane, Queensland, Australia), aligned with the genetic database environment alignment editor, and placed into a phylogenetic tree containing approximately 8,000 rDNA sequences. The neighbor-joining tree was generated on the ARB application (O. Strunk, O. Gross, B. Reichel, M. Max, S. Hermann, N. Struckmann, B. Nonhoff, M. Lenke, A. Vilbig, T. Ludwig, A. Bode, K. H. Schleifer, and W. Ludwig, 1996 [http://www.mikro.biologie.tu-muenchen.de/pub/ARB/documentation/arb.ps]).
Results. PCR was used to amplify 16S rDNA from a mixture of genomic DNA isolated from the ear fluid of a 38-year-old female with otitis externa. Primer pairs were designed to detect either most bacterial species (27F-1492R) or all three domains of life (515F-1492R). The bacterium-specific PCR product was cloned, and select clones were sequenced. The majority of 16S rDNA clones sequenced (~700 nucleotides each) were identical to the 16S rDNA sequence of S. intermedius, an organism commonly associated with dogs and other animals. We then collected samples from a canine pet belonging to the female patient and amplified the 16S rDNA from microbial contents of the ear, as well as the chest and back. Canine microbial 16S rDNA was amplified with primers StaphF-1492R to limit the 16S rDNA PCR product mixture to a population of Staphylococcus and related genera for ease of analysis. All clones tested were indeed S. intermedius. The entire swab sample from both human and canine sources was used for PCR, and so there was no sample available for culturing.
Amplification of microbial 16S rDNA by PCR is very sensitive to contamination by microbial DNA in laboratory reagents and solutions (24, 27). Therefore, we analyzed negative controls by two methods. First, a control was prepared without the addition of sample (i.e., ear fluid) and worked up in the same manner as the true sample. Second, a control was performed during PCR that lacked input genomic DNA. No amplification was observed in these negative controls. We performed RFLP analysis to provide more informative data on the population of bacteria in the otitis externa sample. An analysis of 20 samples indicated a population dominated by Staphylococcus species, particularly S. intermedius (RFLP type A, 50%). The other staphylococcal species present was identified as Staphylococcus capitis (RFLP type B, 15%)
a species
related to S. epidermidis and a common inhabitant of the
human integument (17). Additional species identified
included Pseudomonas sp., Corynebacterium
tuberculostearicum, and Dolosigranulum pigrum (RFLP
types C, D, and E, respectively). All the sequenced 16S rDNA clones
were 99% or greater in identity to known species except for the
unknown Pseudomonas sp. (98% identity to Pseudomonas
fluorescens).
To examine the similarities between S. intermedius from the
human ear and the canine ear, we compared their 16S rDNA sequences with
an external sample of S. intermedius isolated from a canine furuncle (ATCC 51874). Comparison of the 16S rDNA (1,211 nucleotides) of S. intermedius isolated in this study from both the human
and the canine indicated that the sequences were identical at every position to the sequence of the ATCC sample. These sequences were nearly identical to the GenBank entry D83369, except at position 1260. Sequence D83369 has an A at position 1260, whereas the sequences from
this study have a G.
In an attempt to identify greater heterogeneity, we amplified and
cloned the 16S-23S intergenic spacer (ITS) of S. intermedius from the three samples. Primers ITS1F-ITS1R amplified the ITS (390 nucleotides long) and portions of the 16S and 23S rDNA genes (Table 1).
However, the sequences were nearly identical in all three samples. One
region at position 242 of the ITS differed in several clones examined.
At this position, we observed a G-to-A change when comparing the
samples from this study to the sample from the ATCC. A variable
nucleotide at position 50 in the 23S rDNA changed from a U in the human
and canine ear samples to a C in the ATCC sample.
The female patient was treated with the topical antibiotics neomycin
and polymyxin B and quickly recovered from the otitis externa
(approximately 4 days). We again tested an ear sample by PCR to examine
the organisms present in the healthy ear. Examination of the 16S rDNA
sequences indicated that the ear still harbored S. capitis
and Pseudomonas sp., but S. intermedius was absent.
Discussion.
Molecular phylogenetic approaches based on PCR and
sequence analysis of the 16S rRNA gene were applied to identify the
bacteria present in an ear fluid sample from a patient with otitis
externa. We analyzed 25 cloned 16S rDNA sequences from the
27F-1492R-amplified PCR product and noticed an undiversified population
of bacteria dominated by one type. This bacterium was identified as
S. intermedius, an organism normally associated with healthy
animals but capable of causing disease in both animals and humans
(2, 4, 6, 8, 26). S. intermedius is related to
staphylococcal species isolated from various animals and falls into a
cluster referred to as the Staphylococcus hyicus-intermedius
cluster group (Fig. 1) (20).
Four additional species were identified as S. capitis, Pseudomonas sp., C. tuberculostearicum, and
D. pigrum. S. capitis and C. tuberculostearicum
are inhabitants of the human integument and likely represent the normal
microbiota. D. pigrum, isolated originally from the spinal
cord of a multiple sclerosis patient and the eye swab from a neurotopic
cornea (1), appears to be rare, as few descriptions exist.
However, out of 25 clones, the D. pigrum 16S rDNA sequence
was identified only once, and thus it is probably a minor component of
the bacterial population. Interestingly, D. pigrum is
closely related to the genus Alloiococcus, which includes
the single species Alloiococcus otitis, a bacterium identified as a commensal frequently found in ear samples from healthy
adults (data not shown).
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Nucleotide sequence accession numbers. The rDNA sequences of select clones from this study have the GenBank accession no. AF193881 to AF193888.
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ACKNOWLEDGMENTS |
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We thank Edward Bylina, William Coleman, and Mary Yang for comments on the manuscript.
This work was supported by grant R43GM60209-01 from the National Institutes of Health and by Office of Basic Energy Research grant 99ER20211 from the U.S. Department of Energy.
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FOOTNOTES |
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* Corresponding author. Mailing address: Kairos Scientific Inc., Bldg. 62, 3350 Scott Blvd., Santa Clara, CA 95054. Phone: (408) 567-0400, ext. 150. Fax: (408) 567-0440. E-mail: mtanner{at}kairos-scientific.com.
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