Salmonella and antibiotic resistance


There have been recent reports in Europe and elsewhere of the spread of strains of Salmonella that have acquired resistance to different classes of antibiotics. In 2017, 91,662 cases of human salmonellosis (19.7 cases/100,000 population) were confirmed in 37 European countries (including 28 European Union member countries and nine non-member countries), a fairly stable trend from previous years. Recall that salmonellosis, in Europe, is the second largest zoonosis after campylobacteriosis.

Editor’s introductory note. This article aims to offer an update about the development of research on some bacteria resistant to different classes of antibiotics. Salmonella is, moreover, inactivated at a temperature of 70°C. Therefore, it poses no risk of food poisoning on foods intended for consumption after proper cooking. See in this regard the previous article.

Salmonella resistant to different classes of antibiotics

Salmonella is a pathogenic microorganism capable of infecting humans and animals, with a great potential capacity for dissemination in both livestock and food derived from them. The most frequently affected animal species are poultry (broilers and fattening turkeys, as well as laying hens) and pigs. And zootechnical practices-which over time have abused the administration of antimicrobials to animals, not only for therapeutic but mainly prophylactic purposes-have unfortunately favored the selection of resistant bacterial strains.

A carrier of multi-antibiotic resistance (MDR, Multiple Drug Resistance) is defined as a microorganism that is resistant to at least three classes of antibiotics, out of the nine that are tested according to international guidelines. Reduced susceptibility is assessed according to criteria that compare the microorganisms under examination with bacteria that do not show changes in susceptibility to antibiotics (so-called wild-type bacteria). Any deviation from fully susceptible bacteria is taken into account by EUCAST(European Committee on Antimicrobial Susceptibility Testing) to identify potential antimicrobial resistance.

A ‘dangerously’ antibiotic-resistant bacterium is defined as such based on its MDR. To get an idea, in 2016, 26.5 percent of Salmonella strains isolated from humans were MDR, with particularly high resistances to sulfonamides, ampicillin, and tetracyclines. Prominent in this context is Salmonella Kentucky, a serovariant that is not yet well enough known but certainly fearsome, in which MDR is extremely high (76%). According to the most recent EFSA and ECDC report on foodborne zoonoses in Europe, Salmonella Kentucky is seventh in order of frequency as the culprit of human salmonellosis (617 cases in 2017). Its marked resistance to antibiotics can worsen the clinical picture in individuals requiring therapeutic treatment.

Salmonella Infantis
is another serovariant that is distinguished by marked antibiotic resistance. Ranking fourth in Europe in 2017 as the cause of human salmonellosis (1,805 cases), S. Infantis is isolated mainly from broiler chicken, where it is now the most common serovar. As early as 2011, a clonal line of S. Infantis resistant to several classes of antibiotics, such as tetracyclines, sulfonamides, fluoroquinolones, diaminopyrimidines, and third-generation cephalosporins, has been reported in Italy. This MDR clone is also referred to as the ‘Italian broiler chicken clone’ and is widely established to have spread to humans through the consumption of poultry meat.

Salmonella Typhimurium
ranks second among human salmonellosis agents in Europe (10,593 cases in 2017) and shows resistance to critical antibiotics used in human medicine to treat infections with multi-resistant bacteria. One of them is colistin, against which resistant S. Typhimurium strains isolated from pigs, cattle and poultry and their meat are emerging.

However, antibiotic resistance in Salmonella isolated from animal sources and their meat has been widely demonstrated, even beyond the serovars on which microbiologists and clinicians focus. Resistance mainly involves widely used antibiotics, such as quinolonics (nalidixic acid and ciprofloxacin), sulfa drugs, tetracyclines, ampicillin and trimethoprim. Fortunately, resistance to the third-generation cephalosporins, chloramphenicol, colistin, tigecycline, gentamicin, azithromycin, and meropenem remains low at the European level, largely due to no or reduced use in animal husbandry.

The spread of antibiotic resistance-and especially MDR-in zoonotic bacteria such as Salmonella unfortunately looms as an emerging risk that can only be addressed by reducing the use of antimicrobials in farm animals. As well as improving delivery methods (by targeting treatments to individual animals, rather than mass treatments), increasing vaccination strategies, and prioritizing on-farm biosecurity measures. In line with what was dictated in Parliament’s resolution 25.10.18.

Silvia Bonardi


European Food Safety Authority (EFSA), European Centre for Disease Prevention and Control (ECDC) (2018). The European Union summary report on trends and sources of zoonoses, zoonotic agents and food-borne outbreaks in 2017. EFSA Journal 16 (12):5500

European Food Safety Authority (EFSA), European Centre for Disease Prevention and Control (ECDC) (2018). The European Union summary report on antimicrobial resistance in zoonotic and indicator bacteria from humans, animals and food in 2016. EFSA Journal 2018;16(2):5182

Figueiredo R. et al. (2016). Detection of an mcr-1-encoding plasmid mediating colistin resistance in Salmonella enterica from retail meat in Portugal. J Antimicrob Chemother doi:10.1093/jac/dkw240

Franco A. et al. (205) Emergence of a Clonal Lineage of Multidrug-Resistant ESBL-Producing Salmonella Infantis Transmitted from Broilers and Broiler Meat to Humans in Italy between 2011 and 2014. PLoSOne Dec 30;10(12):e0144802

Silvia Bonardi

Graduated in Veterinary Medicine and Specialist in Inspection of Food of Animal Origin and in Veterinary Public Health, she is Professor of Inspection and Control of Food of Animal Origin at the University of Parma.