The More You Know: Brucella and Laboratory Safety

Did you know that despite zoonotic transmission of Brucella spp. within United States being rare, Brucella is one of the most common laboratory transmissible infections?

This is why is important to be aware that on February of 2019 the Center of Disease Control and Prevention (CDC) emitted a food safety alert regarding potential exposures to Brucella RB51 in 19 states (

The diagnosis of brucellosis can be a challenge if it is not suspected due to many reasons. Brucella spp. can be easily aerosolized and the infective dose to cause disease is very low. Clinically, the symptoms can present from 5 days to 6 months after exposure. The acute symptoms are flu-like and non-specific. Although brucellosis is rarely a deadly disease, it can cause chronic infections with multiple complications including arthritis, epididymitis/orchitis, hepatosplenomegaly, endocarditis, and CNS disease.  Despite completing the recommended treatment course of antibiotics, 5-10% of patients with brucellosis have relapse of disease and need additional antibiotic therapy.

In the laboratory, identification of Brucella is also a challenge: they appear as gram negative coccobacilli on Gram stain.  On solid agar they form small (1-2 mm in diameter) smooth and glistening colonies after 24 to 72 h of aerobic incubation. Biochemically they are oxidase, catalase, and urease positive. But there are some caveats: Brucella can stain gram variable or even gram positive (1).  Also, Brucella is one of the few gram negative bacteria that grows well on Colistin-Nalidixic Acid (CNA) agar, so it can be easily mistaken for a gram positive organism. Because of its slow-growing nature, performing biochemical tests to rule Brucella too early can yield false-negative results due to the paucity of organism tested.

The most common reasons for accidental Brucella exposure in the laboratory are lack of experience working with the organism and work performed on an unsuspected Brucella isolate on an open bench.  Not only is the person working with the isolate is exposed, but all those whom are within 5 feet due to aerosolized particles.  The whole laboratory section can be potentially infected depending of the aerosolizing event (

If you work in a sentinel microbiology laboratory and think an isolate may be Brucella, you are advised to perform rule out biochemical tests (oxidase, catalase, and urea) in a biosafety hood using proper PPE and proper disposal techniques for items contaminated with the organism (glass slides, loops, etc).  Keep manipulation of the organism to a bare minimum and do not set up testing such as MALDI-TOF MS, API, or any additional biochemicals until Brucella can be ruled out.  Brucellosis is a mandatory reportable disease, and sentinel laboratories are required to send isolates that are suspicious for Brucella to their local public health departments instead of trying to pursue identification themselves.

What about MALDI-TOF, you may be thinking? Glad you asked! There are methods to inactivate highly infectious pathogens such as Brucella before utilization of MALDI-TOF, but this is not recommended for sentinel laboratories.  Definitive identification is best left to public health and other specialty laboratories with enhanced facilities for working with highly infectious pathogens. In addition, some MALDI-TOF MS databases do not contain spectra for Brucella spp., making it likely that testing will give you no identification or an organism misidentification. 

There are several tests to diagnose brucellosis and serology is a common method.  The CDC requirements for definitive diagnosis are (1) identification from culture or (2) Brucella antibody titers demonstrating a four-fold or greater rise in two weeks or more.  Presumptive diagnosis is defined as (1) Brucella detection by PCR, (2) Brucella total antibody titers of ≥ 1:160 by standard tube agglutination test (SAT), or (3) Brucella microagglutination test (BMAT) in one or more serum specimens.

Now to the present food safety alert: the causative agent is Brucella strain RB51 (RB51), a live-attenuated vaccine for cattle. RB51 can be shed in the milk and people can contract brucellosis after ingestion of raw or unpasteurized dairy products. The presence of RB51 infection is not detected by serological tests and the RB51 stain is resistant to rifampin. This is problematic in two ways: (1) for people exposed to RB51, serology cannot be relied upon for diagnosis and (2) doxycycline plus rifampin for six weeks is one of the most common drug regimens for treatment of brucellosis, but due to rifampin resistance this regimen is not optimal therapy against the RB51 strain of Brucella. 


  1. West, Tsubasa, Rhonda A. Warren, and Siu-Kei Chow. “Photo Quiz: A 66-Year-Old Man with Bloodstream Infection and Back Pain.” (2019): e00381-18.
  2. National Center for Emerging, Zoonotic and Infectious Disease. Centers for Disease Control and Prevention. Brucellosis Reference Guide: Exposures, Testing, and Prevention. February 2017.
  3. Food safety alert: Exposures to Drug-Resistant Brucellosis Linked to Raw Milk, February 8, 2019.
  4. Third Case of Rifampin/Penicillin-Resistant Strain of RB51 Brucella from Consuming Raw Milk
  5. Solera J. Update on brucellosis: therapeutic challenges. Int J Antimicrob Agents. 2010;36:18–20.

-Dennise E. Otero Espinal, MD is a Medical Microbiology Fellow at the University of Chicago (NorthShore).

-Erin McElvania, PhD, D(ABMM), is the Director of Clinical Microbiology NorthShore University Health System in Evanston, Illinois. Follow Dr. McElvania on twitter @E-McElvania. 

One thought on “The More You Know: Brucella and Laboratory Safety”

  1. Hello!

    We recently encountered a Brucella laboratory exposure. Can Brucella spread through serum and plasma samples? Does serology staff need to be screened for Brucella exposure?

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