III.2. Microorganisms that have shown marked resistance
Salmonella, apart from being a bacterium that causes food-borne zoonoses in humans, is one of the responsible for infectious toxic avian hepatitis.
According to a study carried out in China in 2008 with samples from birds in incubators, farms and slaughterhouses, there are multiple strains resistant to doxycycline (see Figure 1, source: ).
The percentage of S. enteritidis resistant strains is lower than in S. indiana (56.9% and 97.85%, respectively). No significant differences were observed in microorganisms obtained from different sources (farm, incubators or slaughterhouse).
b) Streptococcus gallolyticus
Streptococcus species cause opportunistic infections in birds. Signs vary by species, but common presentations include septicemia, peritonitis, salpingitis, and endocarditis.
There is a high prevalence of resistance to doxycycline in strains of S. gallolyticus obtained from birds, even in healthy birds, probably due to the administration of this antibiotic to animals that do not show signs of disease (see Figure 2) .
c) Clostridium perfringens
It is responsible for necrotic enteritis, one of the most important enteric diseases in broilers. It has demonstrated high levels of resistance to doxycycline (98%) in broiler isolates .
 Y. Lu et al., “Prevalence of antimicrobial resistance among Salmonella isolates from chicken in China,” Foodborne Pathog Dis, vol. 8, no. 1, pp. 45–53, 2011.
 R. Nomoto, L. H. Thuy Tien, T. Sekizaki, and R. Osawa, “Antimicrobial susceptibility of Streptococcus gallolyticus isolated from humans and animals,” Jpn. J. Infect. Dis., vol. 66, no. 4, pp. 334–336, 2013.
 K. M. Osman and M. Elhariri, “Antibiotic resistance of Clostridium perfringens isolates from broiler chickens in Egypt,” Rev. sci. tech. Off. int. Epiz, vol. 32, no. 3, pp. 841–850, 2013.