The strain of XDR Shigella sonnei, found in 3 men, is distinct from other globally circulating strains.
Researchers also observed an increase in Shigella infections among people experiencing homelessness.
Researchers estimate vaccines for enteric pathogens could prevent up to 8 to 12 courses of antibiotics per 100 vaccinated children per year in low-resource countries.
Studies conducted in 29 African countries showed the overall pooled prevalence of Shigella bacteria was 5.9%, with concerning levels of antibiotic resistance.
With the pipeline for new antibiotics weak and underfunded, vaccines are seen as a potential tool for preventing the infections that drive antibiotic use and resistance.
The strain of Shigella sonnei is non-susceptible to several classes of antibiotics, and health officials say it's likely to spread.
Montana's H1N2v case was reported in a patient who had attended an agricultural fair.
Of 300 cases recorded since April, 106 have been in the US, with the rest in Europe.
The study authors suggest the strain's resistance to ceftriaxone is likely being driven by bystander resistance resulting from the use of ceftriaxone for treatment of gonorrhea.
Officials have received a growing number of reports of XDR Shigella, which is highly transmissible and resistant to commonly recommended antibiotics, in adults.
