A 4th case of a superbug that is resistant to antibiotics of last resort has been reported in a Connecticut toddler, and health officials are scrambling to figure out where else the superbugs might be lurking.
In both cases, the bacteria were found to be carrying mcr-1, a gene that makes bacteria resistant to antibiotics.
Mcr-1 allows the organism to withstand colistin, the antibiotic doctors turn to when all others have failed. 
In the Connecticut case, the E. coli strain was resistant to colistin but was susceptible to other antibiotics, according to the CDC report. The child fully recovered, and the bacteria did not spread to any of the girl’s family members or to the healthcare providers she came in contact with.
But the case was startling enough that officials said to expect more cases like the toddler’s, and recommended increased surveillance for bacteria that show resistance to colistin.
Details of the Connecticut Case
The toddler had traveled with her family to the Caribbean for about two weeks to visit friends and relatives. During her visit, she ate chicken and goat meat from a live animal market, and developed fever and bloody diarrhea on June 12, two days before returning to the United States.
Upon her return home, doctors collected stool samples from the child, including one from a soiled diaper. An investigation by state and federal health officials determined it wasn’t the strain of E. coli that made her sick. However, the bacteria were discovered to be carrying the mcr-1 gene.
Alexander Kallen, a CDC medical officer who investigates antibiotic resistance, said “It was completely an incidental finding.”
The History of mcr-1
I’ve been following the spread of antibiotic-resistant bacteria for nearly a year now. It first popped up in China in November 2015. Experts warned then that the world might soon be coping with a serious superbug problem, and predicted that medicine could very well return to the Dark Ages.
From there, the mcr-1 gene showed up in bacteria in Denmark, followed by the United Kingdom. Experts thought it would take about a decade for antibiotic-resistant bacteria to reach England’s shores, but it took mere months.
Health officials in Canada announced in January 2016 that antibiotic-resistant bugs had been discovered in that country in old samples dating back to 2010 and 2011. That finding was especially alarming because it indicated that the bacteria had been in North America much longer than anyone realized.
The resistance gene was also found in an E. coli strain in a 76-year-old New Jersey man dating back to August 2014, Rutgers University scientists reported last month, leading to the same conclusion.
In the case of the Pennsylvania woman, health officials in that state contacted everyone who had been near the infected patient, but none of them had any known risk factors for mcr-1.
Kelly Kline of the Pennsylvania Department of Health and colleagues wrote:
“It is not known how the patient became colonized, especially in the absence of an epidemiologic link to known persons or places with identified mcr-1.
The patient had no international travel for approximately one year, no livestock exposure and a limited role in meal preparation with store-bought groceries; however, she had multiple and repeated admissions to four medical facilities during 2016.” 
Health care personnel from two high-risk facilities where the Pennsylvania woman was treated were tested, plus 20 people who had high-risk contact, including a hospital roommate, family and friends, and nurses who bathed her. No bacteria with the mcr-1 gene were detected among the 105 persons screened.
Like the Connecticut patient, the Pennsylvania patient didn’t appear to have been sickened by E. coli. She had other infections and wasn’t treated for the E. coli infection. When a person carries bacteria that don’t sicken them, it’s called “colonization.”
On August 1, the patient tested negative for the mcr-1 carrying gene.
Where We’re Headed
What makes mcr-1 so frightening is the fact that it’s found on a plasmid, meaning that it can affect many different kinds of bacteria.
The worst-case scenario – the thought that keeps researchers and health officials up at night – is that the mcr-1 gene will spread to another superbug with other mutations. The result would be a super-superbug invincible to every life-saving antibiotic on the planet.
In the United States, colistin is not typically used to treat humans, nor is it used in livestock. However, colistin use among farm animals is widespread in other countries, particularly China, which is the primary reason bacteria are becoming resistant to it.
The world needs new antibiotics, but antibiotics aren’t money-makers for drug companies.
 NBC News