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• Laboratory Diagnosis of Infectious Disease
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Culture is microbial growth on or in a nutritional solid or liquid medium; increased numbers of organisms simplify identification. Culture also facilitates testing of antimicrobial susceptibility.

Communication with the laboratory is essential. Although most specimens are placed on general purpose media (eg, blood or chocolate agar), some pathogens require inclusion of specific nutrients and inhibitors or other special conditions (see Table 1: Laboratory Diagnosis of Infectious Disease: Selective Media for Isolation of Common Bacteria); if one of these pathogens is suspected or if the patient has been taking antimicrobials, the laboratory should be advised. The specimen's source is reported so that the laboratory can differentiate pathogens from site-specific normal flora.

Table 1
Selective Media for Isolation of Common Bacteria Organism Preferred Medium Bacteroides sp KanamycinSome Trade Names KANTREX Click for Drug Monograph -vancomycinSome Trade Names VANCOCIN Click for Drug Monograph laked blood agar Bacteroides fragilis Bacteroides bile-esculin (with gentamicinSome Trade Names GARAMYCIN Click for Drug Monograph and bile) Bordetella pertussis Bordet-Gengou agar plus methicillin or cephalexinSome Trade Names KEFLEX KEFTAB Click for Drug Monograph Regan-Lowe cephalexinSome Trade Names KEFLEX KEFTAB Click for Drug Monograph agar Horse blood–charcoal agar Burkholderia cepacia Pseudomonas cepacia agar Campylobacter jejuni or C. coli Campylobacter-selective agars (eg, cefoperazone-vancomycinSome Trade Names VANCOCIN Click for Drug Monograph agar) Corynebacterium diphtheriae Tinsdale agar Cystine-tellurite blood agar Löffler coagulated serum medium Escherichia coli or enterohemorrhagic pathogens (Shiga toxin producers, including O157-H7) MacConkey-sorbitol agar used with specific antisera Francisella tularensis Blood- or chocolate-cystine agar Legionella sp Buffered charcoal yeast extract agar Leptospira sp Fletcher's or Stuart's medium with rabbit serum or Leptospira medium with bovine serum albumin-Tween 80 Neisseria gonorrhoeae or N. meningitidis Modified Thayer-Martin agar New York City agar Salmonella and Shigella sp May grow on standard MacConkey or eosin-methylene blueSome Trade Names UROLENE BLUE Click for Drug Monograph Alternative: Hektoen or xylose-lysine-desoxycholate, Salmonella-Shigella agar, gram-negative or selenite enrichment broth Vibrio sp Thiosulfate-citrate-bile salts–sucrose agar Yersinia sp Cefsulodin-Irgasan-novobiocin agar

Specimen collection is important. The wrong type of swab can produce false-negative results. Wooden-shafted swabs are toxic to some viruses. Cotton-tipped swabs are toxic for some bacteria and chlamydiae. Blood cultures require decontamination and disinfection of the skin (eg, povidone iodine swab, allowed to dry, removed with 70% alcohol). Multiple samples, each from a different site are generally used; they are taken nearly simultaneously with fever spikes if possible. Normal flora of skin and mucous membranes that grows in only a single blood sample is usually interpreted as contamination. If a blood specimen is obtained from a central line, a peripheral blood specimen should also be obtained to help differentiate systemic bacteremia from catheter infection. Cultures from infected catheters generally turn positive more quickly and contain more organisms than simultaneously drawn peripheral blood cultures. Some fungi, particularly molds (eg, Aspergillus sp), usually cannot be cultured from blood.

The specimen must be transported rapidly, in the correct medium, and in conditions that limit growth of any potentially contaminating normal flora. For accurate quantification of the pathogen, additional pathogen growth must be prevented; specimens should be transported to the laboratory immediately or, if transport is delayed, refrigerated (in most cases).

Certain cultures have special considerations.

Anaerobic bacteria should not be cultured from sites where they are normal flora because differentiation of pathogens from normal flora may be impossible. Specimens must be shielded from air, which can be difficult. For swab specimens, anaerobic transport media are available. Specimens collected with a syringe (eg, abscess contents) should be transported in the syringe.

Mycobacteria are difficult to culture. Specimens containing normal flora (eg, sputum) must first be decontaminated and concentrated. Mycobacterium tuberculosis and some other mycobacteria grow slowly. Growth of M. tuberculosis is typically faster in liquid than in solid media; routine use of automated systems with liquid media can result in growth within 2 wk vs ≥ 4 wk on solid media such as Lowenstein-Jensen agar. In addition, few organisms may be present in a specimen. Multiple specimens from the same site may help maximize yield. Specimens should be allowed to grow for 8 wk before being discarded. If an atypical mycobacterium is suspected, the laboratory should be notified.

Viruses are generally cultured from swabs and tissue specimens usually transported in media that contain antibacterial and antifungal agents. Specimens are inoculated onto tissue cultures that support the suspected virus and inhibit all other microbes. Viruses that are highly labile (eg, varicella zoster) should be inoculated onto tissue cultures within 1 h of collection. Standard tissue cultures are most sensitive. Rapid tissue cultures (shell vials) may provide more rapid results. Some common viruses cannot be detected using routine culture methods and require alternative methods for diagnosis (eg, enzyme immunoassay for Epstein-Barr virus, hepatitis B and E viruses, HIV, and human T-lymphotropic virus; serologic tests for hepatitis A and D viruses; nucleic acid–based methods for HIV).

Table 2
Common Viruses That Do Not Grow in Routine Viral Cultures Common Conditions Virus Diagnostic Tests Acute febrile illness, meningoencephalitis Alphavirus, flaviviruses, bunyaviruses (eg, St. Louis encephalitis virus, La Crosse encephalitis virus) EIA Diarrhea Rotaviruses, caliciviruses (noroviruses), astroviruses EM or IEM Infectious mononucleosis Epstein-Barr virus EIA Hemorrhagic fevers, lymphocytic choriomeningitis Filoviruses, arenaviruses (eg, Lassa fever, Ebola virus) EM Hepatitis Hepatitis A, hepatitis D Serologic testing Hepatitis B, hepatitis E EIA Hepatitis C, hepatitis G Nucleic acid–based methods, EIA Roseola, Kaposi's sarcoma, disseminated infections Herpesviruses 6, 7, 8 Nucleic acid–based methods, EIA AIDS HIV Nucleic acid–based methods, EIA Condylomata acuminata, genital skin cancer Human papillomaviruses Nucleic-acid–based methods, EIA Fifth disease Human parvovirus B19 Nucleic acid–based methods, EIA Adult T-cell leukemia Human T-lymphotrophic virus EIA Progressive multifocal leukoencephalopathy, kidney infection Polyoma viruses (JC and BK) Nucleic acid–based methods Smallpox, monkeypox, vaccinia, molluscum contagiosum Poxviruses Nucleic acid–based methods, EM, culture depending on virus Rabies Rabies virus EM, IFA Rubella Rubella virus EIA, IFA EIA = enzyme immunoassay; EM = electron microscopy; IEM = immunoelectron microscopy; IFA = immunofluorescence assay.

Fungi specimens obtained from nonsterile sites must be inoculated onto media containing antibacterial agents. Specimens should be allowed to grow for 4 wk before being discarded.

Last full review/revision September 2009 by Kevin C. Hazen, PhD

Content last modified September 2009