According to the Centers for Disease Control and Prevention (CDC), Salmonella causes an estimated one million illnesses in the United States alone, including 19,000 hospitalizations and 380 deaths (Salmonella) . Children under 5 are more likely to become infected than adults and are more likely to acquire severe infections .
State health departments and the CDC generally use serotyping and Pulsed-field Gel Electrophoresis (PFGE) to assess strains of bacteria. Serotyping is based on unique surface structures , whereas PFGE uses electric fields to separate DNA fragments based on their size and create a DNA fingerprint . State, local, and federal agencies share PFGE results through a database maintained by PulseNet .
Multi locus sequence typing (MLST) is another method of strain analysis. It is used to identify allelic differences in the sequence of various house-keeping genes . The differences in genes may or may not correspond to the serotype of Salmonella. MLST involves amplification of housekeeping genes by PCR, then sequencing and interrogation of the sequences against a database to determine sequence type .
A recent nosocomial outbreak of Methicillin-resistant Staphylococcus aureus (MRSA) at a hospital in West Virginia left employees concerned when two cases of Salmonella presented themselves at the same hospital within a short period of time. The following case study investigates two cases of infantile Salmonella infection. MLST was used to confirm any relation between the two cases and then compared to serotyping and PFGE results from the West Virginia state health department.
A male, born 11 Nov 2014, with chromosomal anomalies was admitted to the NICU in January, 2015, due to respiratory distress. Ventilator support was required. The patient had an elevated white blood cell count with a left shift, increased C-reactive protein levels, anemia, and electrolyte disturbances. Due to the likelihood of infection, broad spectrum antibiotics were administered.
During the patient’s hospital stay, he was weaned off of ventilator support but continued to have intermittent fever, leukocytosis, and pyuria. Radiography suggested a renal lesion, possibly a fungus ball but urine culture performed on 1 March 2015 was positive for Salmonella sp. Subsequent blood and stool cultures failed to recover the organism. The patient was diagnosed with salmonellosis in early March.
The patient received 10 days I.V. cephalosporin therapy infection was after which complete clearance was documented. The patient expired in late May due to underlying medical problems.
A 1 day old male was admitted to the NICU in April, 2015, due to prematurity and gastroschisis. The patient underwent surgical repair and Broviac catheter placement for total parenteral nutrition. He was discharged two months later, at the beginning of June, following catheter removal, normal food intake, and adequate weight gain.
The patient was readmitted to the PICU, one week after discharge, with possible sepsis and history of watery diarrhea. Vancomycin, gentamicin, and metronidazole were administered. A stool culture was positive for Salmonella. After a 14 day course of ampicillin/sulbactam, the patient was released fully recovered.
Hospital acquired infection and potential nosocomial transmission was a concern for both cases. To rule out any association between the two cases, Multi-locus Sequence Typing (MLST) was used to assess the isolate from each case. The resulting sequence type for case 1 was ST-1498 and for case 2 was ST-19.
Serotyping at the West Virginia Office of Laboratory Services (WV-OLS) demonstrated that case 1 was S. wandsworth and for case 2 S. typhimurium. These serotypes correlate with sequence types 1498 and 19, respectively, according to unique surface structures.
Results of PFGE performed at WV-OLS were compared for the two cases in order to corroborate results from MLST. Figure 1 shows the DNA fingerprints generated by PFGE. Case 1 is represented by number 1 and case 2 is represented by number 21. There are clearly >2 band differences between the two fingerprints which confirms the MLST results.
Figure 1. Pulsed Field Gel Electrophoresis results for cases
From the MLST and PFGE results, it can be concluded that the two cases were isolated incidences. For case 1, the patient never left the hospital but it is unclear if the infection was the result of the hospital or was carried in by a relative from outside the hospital. For case 2, the infant left the hospital for 7 days before returning with symptoms. Since the incubation period of Salmonella infection is only 12-36 hours , it is likely that the illness was acquired outside of the hospital.
The hospital setting can be a major contributor to acquisition of infectious diseases for several reasons: hospital patients are frequently on immunosuppressant drugs, various co-morbidities exist among hospitalized persons, immature immune systems are common among neonates, and the elderly and hospitalized patients are at higher risk of exposure to virulent, multi-drug resistant organisms. In a review of 52 nosocomial Salmonella outbreaks, foodborne transmission was the most common source of infection (59.6%), followed by person-to-person transmission (13.5%) and other (5.8%), which included contaminated equipment and transfer for home environment. The remaining outbreaks did not report a source .
One study investigating an outbreak of S. typhimurium in a pediatric ward in South Africa in 2012 reported 22 cases at the peak of the outbreak . Of the 22 cases, 4 were HIV positive and 11 others had problems suggesting an immunocompromised state. The median age of patients was 11 months. The outbreak was attributed to high person to person transmission due to poor handwashing and hygiene, suboptimal infection control practices, hospital ward overcrowding, and low ratio of nurses to patients . The significance of this study in relation to the study done above is that it addresses the importance of monitoring Salmonella in pediatric wards, the ease with which it spreads under such circumstances, and the reason for concern when two patients acquired salmonellosis within a short time period.
Two cases of neonatal salmonellosis were investigated by isolate serotyping and by multi locus sequence typing (MLST). The two infants were admitted to the same hospital unit several months apart and developed what appeared to be hospital acquired Salmonella infection. An isolate from each patient was serotyped and underwent MLST to establish any relation between the two cases. The resulting sequence types were 1498 for case 1 and 19 for case 2. From these results, it can be concluded that the two cases were not related. The results matched results from the local state health department, which were S. wadsworth for case 1 and S. typhimurium for case two, and confirmed the isolates were not related.
The authors would like to thank Dr. Charles Mullett of the PICU in the Department of Pediatrics for his helpful feedback on this manuscript.
No funding was received.
Conflict of interest
The authors have no conflicts of interest to declare.