Peptostreptococcus Infection Workup

Updated: Apr 30, 2021
  • Author: Itzhak Brook, MD, MSc; Chief Editor: Michael Stuart Bronze, MD  more...
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Laboratory Studies

Microbiology  [10]

Anaerobic, microaerophilic, and facultative gram-positive cocci have minor morphological differences. P magnus has a larger diameter than other anaerobic gram-positive cocci. P micros has a smaller diameter than other anaerobic gram-positive cocci and usually forms short chains. P anaerobius and Peptostreptococcus productus are elongated and often appear in pairs or chains.

Gas-liquid chromatography and biochemical tests are required for genus-level identification and separation of most anaerobic gram-positive cocci.Modern molecular techniques, such as 16s PCR and pyrosequencing can identify these organisms. These organisms are fastidious, and their complete identification is often difficult. Because of ill-defined differences in the pathogenic potential for the different species, the need for exact specification is controversial.

Anaerobic cocci show slow but adequate growth on all nonselective anaerobic growth media. Vancomycin-containing selective media inhibit their growth.

Recovery in clinical specimens  [10]

Anaerobic and facultative gram-positive cocci are often isolated from clinical specimens mixed with other anaerobic or aerobic bacteria and, on rare occasions, are isolated as the sole pathogen. As a group, these organisms are the most frequently recovered anaerobes in cutaneous, oral, respiratory tract, and female genital tract infections.

Collecting anaerobic bacteria specimens is important because documentation of an anaerobic infection is through culture of organisms from the infected site. Documentation requires proper collection of appropriate specimens, expeditious transportation, and careful laboratory processing.

Obtain uncontaminated specimens. Inadequate culture techniques or media can lead to faulty results and the incorrect conclusion that only aerobic organisms are present in a mixed infection. Specimens must be obtained free of contamination. Inadequate techniques or media can lead to missing the presence of anaerobic bacteria or the assumption that only aerobic organisms are present in a mixed infection.

Because anaerobes are present on mucous membranes and skin, even minimal contamination with normal florae can be misleading.

Unacceptable or inappropriate specimens can also yield normal florae and therefore have no diagnostic value. Obtain appropriate specimens using techniques that bypass the normal florae.

Direct-needle aspiration is the best method of obtaining a culture. Direct-needle aspiration is probably the best method of obtaining a culture, and the use of swabs is much less desirable. Specimens obtained from normally sterile sites, such as blood, spinal, joint, or peritoneal fluids, are collected after thorough skin decontamination. Two approaches are used to culture the maxillary sinus by aspiration following sterilization of the canine fossa or the nasal vestibule, either via the canine fossa or via the inferior meatus.

Urine collected is collected by percutaneous suprapubic bladder aspiration.

Other specimens can be collected from abscess contents, from deep aspirates of wounds, and by special techniques, such as transtracheal aspirates or direct lung puncture.

Specimens of the lower respiratory tract are difficult to obtain without contamination with indigenous florae. Double-lumen catheter bronchial brushing and bronchoalveolar lavage, cultured quantitatively, can be useful.

Culdocentesis fluid obtained after decontamination of the vagina is acceptable.

Transportation of specimens should be expeditious. Place specimens into an anaerobic transporter as soon as possible. These devices generally contain oxygen-free environments provided by a mixture of carbon dioxide, hydrogen, and nitrogen plus an aerobic condition indicator.

Liquid or tissue specimens are always preferred to swabs.

Inoculate liquid specimens into an anaerobic transport vial or a syringe. All air bubbles are expelled from the syringe. Insertion of the needle tip into a sterile rubber stopper is no longer recommended. Because air gradually diffuses through the plastic syringe wall, specimens should be processed in less than 30 minutes.

Transport tissue specimens in an anaerobic jar or a sealed plastic bag rendered anaerobic.

If swabs are used, place them in sterilized tubes containing carbon dioxide or prereduced, anaerobically sterile Carey and Blair semisolid media.

Gram stain of a smear of the specimen provides important preliminary information regarding types of organisms present, suggests appropriate initial therapy, and serves as a quality control. Immediately place cultures under anaerobic conditions and incubate for 48 hours or longer. An additional 36-48 hours is usually required for species- or genus-level identification using biochemical tests; kits containing these tests are commercially available.

A rapid enzymatic test enables identification after only 4 hours of aerobic incubation. Gas-liquid chromatography of metabolites is often used. Nucleic acid probers and polymerase chain reaction methods are also being developed for rapid identification. [28] Detailed procedures of laboratory methods can be found in microbiology manuals.

Antimicrobial susceptibility test results of peptostreptococci have become less predictable because of the increasing resistance of peptostreptococci to several antimicrobials. [29] Resistance to metronidazole is common. Routine susceptibility testing is time consuming and often unnecessary; however, testing the susceptibility of isolates recovered from sterile body sites, those that are clinically important and have variable susceptibilities, and especially those isolated in pure cultures from properly collected specimens is important. These include isolates associated with bacteremia; endocarditis; and bone, joint, or skull infections. [30]

Routine susceptibility testing is time-consuming and often unnecessary. Poor quality control of in-vitro susceptibility testing, and the difficulty to obtain results in a reasonable time makes testing less useful. However, testing the susceptibility of isolates recovered from sterile body sites and/or those that are clinically important (ie, blood cultures, bone, CNS, serious infections) and have variable susceptibilities, especially those isolated in pure culture from properly collected specimens, is important. Antimicrobial testing is recommended, however, to monitor local hospital and geographic susceptibility profiles, and to determine the activity of new antimicrobials.

Recommended methods include agar microbroth and macrobroth dilution. Newer methods include the E-test and the spiral gradient end point system. Agents that should be tested include penicillin, broad-spectrum penicillin, penicillin plus a beta-lactamase inhibitor, clindamycin, chloramphenicol, second-generation cephalosporins (eg, cefoxitin), moxifloxacin, tigecycline, metronidazole, and carbapenems.


Imaging Studies

Radiological or imaging studies are helpful. The presence of gas in the infected site is a strong indication of anaerobic infection.