CDC's update also noted that the agency's National Antimicrobial Resistance Monitoring System for Enteric Bacteria (NARMS) laboratory continues to conduct antimicrobial susceptibility testing on clinical isolates collected from ill persons infected with all seven of the outbreak strains. The update explained:

Of 34 isolates tested to date, 19 (56%) of these isolates exhibited resistance to one or more antibiotics. Seven (21%) of the 34 isolates were multidrug resistant. To date, isolates collected from ill persons were resistant to combinations of the following antibiotics: ampicillin, chloramphenicol, gentamicin, kanamycin, streptomycin, sulfisoxazole, and tetracycline. Antibiotic resistance may increase the risk of hospitalization or possible treatment failure in infected individuals.

Seven strains of Salmonella Heidelberg bacteria have been identified as being linked to this outbreak. Ill persons infected with each of the seven strains were linked to consumption Foster Farms chicken. Four of these strains are rarely reported to PulseNet. The other three strains are more common, with several ill persons infected with each strain reported to CDC monthly. The number of reported cases for these three strains was significantly higher than the number of cases expected during the outbreak period. Since illnesses due to several strains are more commonly reported, not all may be linked to consumption of Foster Farms chicken and may be part of the expected number of illnesses reported during this period.