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Salmonella and antibiotic resistance

The spread of strains of Salmonella which have acquired resistance to different classes of antibiotics. In 2017, 37 cases of human salmonellosis (28 cases / 91.662 inhabitants) were confirmed in 19,7 European countries (of which 100.000 EU member countries and nine non-member countries), with a fairly stable trend compared to previous years. . Recall that salmonellosis, in Europe, is the second zoonosis after campylobacteriosis.

Editor's introductory note. This article aims to offer an update on the development of research on some bacteria resistant to different classes of antibiotics. Salmonella is also inactivated at a temperature of 70 ° C. Therefore, it does not present any risk of food poisoning on food intended for consumption after adequate cooking. In this regard, see the previous article.

Salmonella resistant to several classes of antibiotics

Salmonella is a pathogenic microorganism capable of infecting humans and animals, with a great potential capacity for dissemination both in livestock and in food derived from them. The animal species most frequently involved are poultry (broilers and turkeys for fattening, 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 above all prophylactic purposes - have unfortunately favored the selection of resistant bacterial stems.

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

A 'dangerously' resistant bacterium antibiotic is defined as such based on its MDR. To get an idea, in 2016 26,5% of Salmonella stems isolated from humans were MDR, with particularly high resistance to sulfonamides, ampicillin and tetracyclines. In this context, it stands out Kentucky Salmonella, a serovariant not yet sufficiently known but certainly fearful, in which the MDR is extremely high (76%). According to the most recent EFSA and ECDC report on foodborne zoonoses in Europe, Kentucky Salmonella it is the seventh in order of frequency as responsible for human salmonellosis (617 cases in 2017). Its marked resistance to antibiotics can worsen the clinical picture in individuals requiring therapeutic treatment.

Salmonella Infantis it is another serovariant which is distinguished by its marked antibiotic-resistance. In fourth place in Europe in 2017 as a cause of human salmonellosis (1.805 cases), St. Infantis it isolates itself above all from broiler chicken, where it is today the most common serovariant. Already starting from 2011, a clonal line of St. Infantis resistant to different classes of antibiotics, such as tetracyclines, sulfonamides, fluoroquinolones, diaminopyrimidines and third generation cephalosporins. This MDR clone is also called the 'Italian broiler clone' and its spread to humans through the consumption of poultry meat is well established.

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 for the treatment of infections by multi-resistant bacteria. One of them is represented by colistin, against which strains of S. Typhimurium resistant isolates from pigs, cattle and poultry and their meat.

Antibiotic resistance in Salmonella isolated from animal sources and their meats, however, it has been widely demonstrated, even beyond the serovariates on which the attention of microbiologists and clinicians is focused. Resistance mainly involves widely used antibiotics, such as quinolones (nalidixic acid and ciprofloxacin), sulfonamides, tetracyclines, ampicillin and trimethoprim. Fortunately, resistance to third generation cephalosporins, chloramphenicol, colistin, tigecycline, gentamicin, azithromycin and meropenem remains low in Europe, largely due to the lack or reduced use in animal husbandry.

The spread of antibiotic resistance - and especially of the MDR - in zoonotic bacteria such as Salmonella unfortunately looms as an emerging risk, which can only be addressed by reducing the use of antimicrobials in livestock. As well as improving the methods of administration (through treatments targeted to individual animals, rather than mass treatments), increasing vaccination strategies and favoring biosecurity measures on the farm. In line with the provisions of the resolution 25.10.18 of the European Parliament.

Silvia Bonardi


European Food Safety Authority (EFSA), European Center 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 Center 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

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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. 

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