How is Salmonella Infection Diagnosed?
Salmonella bacteria can be detected by stool culture. In cases of bacteremia or invasive illness, the bacteria can also be cultured from blood, urine, or, on rare occasions, from other body tissues. A fecal, blood, or other sample is placed in nutrient broth or on agar and incubated for up to 2-3 days. After that time, a trained microbiologist can identify the bacteria, if present, and confirm its identity by looking at biochemical reactions. Typically, this test is conducted at a hospital or clinical laboratory. Isolates obtained from culture are forwarded from clinical laboratories to public health laboratories.
Treatment with antibiotics before collecting a specimen for testing can affect bacterial growth in culture and lead to a negative test result even when Salmonella causes the infection. Thus, it is important to collect a specimen for laboratory testing before antibiotics are given.
Culturing of organisms such as Salmonella has been the mainstay of diagnostic testing conducted at clinical laboratories. Isolates obtained from culture are forwarded from clinical laboratories to public health laboratories, where additional testing is performed, including serotyping, antimicrobial susceptibility, and genetic testing. Results of these additional tests allow epidemiologists to monitor trends and detect outbreaks.
Increasingly, hospitals and clinical laboratories are using culture-independent diagnostic tests (CIDTs) rather than culture-based testing to detect enteric pathogens such as Salmonella. Commonly used CIDT testing methods include testing by polymerase chain reaction (PCR) and enzyme immunoassay (EIA) testing. CIDTs have a rapid turnaround time, can detect multiple pathogens in one test, and cost less than traditional testing procedures such as culture, which often require specialized media and years of training. Clinicians are thus able to identify and provide appropriate treatment in a shorter amount of time.
However, a CIDT-identified positive specimen of Salmonella might not be available for the serotyping and advanced genetic characterization that is used to identify disease clusters and outbreaks. For these activities, a culture-derived isolate is required. Clinical laboratories must then submit CIDT-identified specimens to a public health laboratory, which attempts to replicate clinical laboratory results and recover culture-derived isolates. In one study conducted at the Tennessee Department of Health Public Health Laboratory, isolates were recovered in only 72% of CIDT-identified positive Salmonella specimens. This has important implications for disease surveillance and outbreak detection, particularly since timely reporting of foodborne diseases is necessary to identify persons at risk for exposure and to prevent additional cases in outbreak settings.
There are multiple steps between the onset of a foodborne illness and its investigation by a public health agency, which have the potential to delay recognition of outbreaks caused by reportable enteric diseases. As pointed out by Craig Hedberg et al. in their study published in Emerging Infectious Diseases:
One important way to speed the detection of outbreaks is to encourage clinicians to immediately notify health departments when they suspect a patient is part of an outbreak. Since many outbreaks caused by E. coli O157:H7 and Salmonella spp. last multiple days, physician reporting concurrent with stool collection may provide opportunities for a public health intervention that could prevent outbreak-associated cases.
Several studies have recommended some strategies to increase the rate of positive stool culture: (1) not performing routine cultures in patients who experience the onset of diarrhea three or more days after admission to the hospital, (2) not using multiple specimens (but, instead, using one appropriate specimen), (3) not culturing by smears of rectal swabs, and (4) eliminating stool specimens taken from patients that were treated with antibiotics prior to providing a specimen.