Beware what you eat

Salmonella is one of the top global causes of diarrheal diseases, with over 1 million illnesses each year in the US, almost all cases linked to contaminated or undercooked food. Antibiotic-resistant Salmonella infections in the United States are now 6,200 cases annually, though estimates accounting for underreporting suggest over 100,000 drug-resistant infections per year.

The Pathogen

Salmonella enterica are Gram-negative rod-shaped bacteria that are facultative intracellular pathogens. This means that Salmonella is able to grow inside or outside of host cells. Several subspecies of S. enterica cause disease in humans and are divided into two main groups - typhoidal and non-typhoidal. While typhoidal strains only infect humans and cause typhoid fever, non-typhoidal species may infect either a broad range of vertebrate animals or only infect a particular animal and do not result in typhoid fever.

Salmonellosis

Salmonella has been known to cause illness for 125 years, first discovered by an American scientist Dr. Salmon. Children under the age of 5, the elderly, and anyone with weakened immune systems have a higher risk of infection. The illness caused by a Salmonella infection is called Salmonellosis, primarily caused by ingestion of contaminated food. The majority of salmonellosis cases are caused by non-typhoidal strains, infections with symptoms of diarrhea, abdominal cramps, and a fever. Typhoidal Salmonella cause typhoid fever, a life-threatening disease affecting 22 million people worldwide each year.

Salmonella resistance is on the rise

Salmonella gains resistance to antibiotics by enzymatic hydrolysis of beta-lactams, modification of drug targets (amino acid substitutions in gyrase and other proteins targeted by quinolones), and expression of antibiotic efflux pumps (AcrAB-TolC-type efflux pumps to remove ciprofloxacin). Among typhoidal Salmonella, resistance to traditional first-line antimicrobials ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole is commonly caused by resistance determinants located on plasmids - small DNA molecules separate from chromosomal DNA that can replicate independently and can be shared from one bacteria to another. In the 1970's, chloramphenicol was the treatment of choice, but in 1972 an epidemic was caused by a chloramphenicol-resistant strain, followed by outbreaks across the globe. The resistance determinant was carried on a self-transmissible plasmid that also carried resistance to other drugs such as streptomycin, sulfonamides, and tetracyclines.

Competing for metals in the gut

CHARM Investigators use probiotic bacteria to steal metals from Salmonella

Salmonella can hoard iron and other metals for its own nutritional use, allowing it to thrive in the gut while other bacteria struggle. A non-pathogenic, probiotic strain of E. coli however has a similar technique, allowing it to directly compete for iron. When both bacteria are present in the gut, this fierce competition for metals reduces Salmonella levels in the gut without using antibiotics.

Read the Article from Cell Host & Microbe

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