Cantaloupe listeriosis outbreak: genetic analysis of Listeria strains
What it found
The 2011 US cantaloupe outbreak was caused by several different strains of Listeria monocytogenes, including two serotypes and four PFGE patterns.
The strains were genetically distinct from those in earlier outbreaks.
What they found
Studies in people
Outbreak size and impact
The outbreak caused 146 invasive illnesses, 30 deaths and one miscarriage across 28 states. It was the largest number of fatalities from a foodborne listeriosis outbreak in the United States.
Animal and lab studies
Genetic groups within serotype 1/2a
The 1/2a isolates split into two distinct genomic groups: one represented by pattern I and the other by patterns III and IV. The pattern I strains were genetically closer to strains from a 1983 milk outbreak than to the other cantaloupe strains.
Phage genes in pattern IV
More than 70% of the extra gene sequences found in pattern IV strains were similar to phage-related genes. This suggests that phage may have played a role in separating these strains.
Serotype 1/2b relationships
The 1/2b cantaloupe strains clustered together and were closely related to unrelated strains that had caused invasive listeriosis. They were more distantly related to strains from febrile gastroenteritis outbreaks.
Distinct from earlier outbreaks
The gene content of the cantaloupe outbreak strains was different from that of previously reported outbreak strains of the same serotypes. This shows the outbreak strains were genetically distinct.
Other findings
Multiple strains involved
The outbreak involved two serotypes (1/2a and 1/2b) and four PFGE pattern combinations (I, II, III, IV). This was one of the few listeriosis outbreaks where more than one PFGE profile was linked to the outbreak.
What the authors conclude
“This work underscores the utility of multiple approaches, such as serotyping, PFGE and DNA microarray analysis to characterize the composition of complex polyclonal listeriosis outbreaks.”
Also in their conclusions
- They say the outbreak was caused by several distinct serotypes and genotypes of L. monocytogenes.
- They note that the increasing discriminatory power of serotyping, PFGE and DNA microarray analysis underscores the utility of a multiphasic approach in revealing the complex polyclonal nature of this outbreak.
- They suggest that phage genomes may be an important driving force behind the evolution of some strains of L. monocytogenes.
How it was done
The authors wanted to understand the genetic makeup of the Listeria strains behind the 2011 cantaloupe outbreak, which was the first listeriosis outbreak linked to fresh fruit in the US and the deadliest foodborne listeriosis outbreak in the country.
They analysed 35 outbreak-associated Listeria isolates from patients, cantaloupe and the packing facility environment. They used serotyping, PFGE and a DNA microarray (Listeria GeneChip) to compare the gene contents of 16 representative isolates.
What it can’t tell you
- This study looked at the genetics of the bacteria, not at how sick people got or how much cantaloupe they ate.
- The genetic analysis cannot show whether one strain was more likely to cause severe illness than another.
- The findings are about this specific outbreak and may not apply to other Listeria strains or other foods.
Who paid
- Conflicts
- Competing Interests: The authors have declared that no competing interests exist.
- Authors work at
- Center for Food Safety and Applied Nutrition, United States of America
The paper
- Title
- Genomic characterization of Listeria monocytogenes strains involved in a multistate listeriosis outbreak associated with cantaloupe in US
- Type
- Study
- Evidence
- Studies in people · animals · lab
- Summarised from
- Full text
- Licence
- CC BY. Words in quotation marks are the authors’; the rest is our summary.
- Cite
- Laksanalamai P, Joseph LA, Silk BJ, et al (2012). Genomic characterization of Listeria monocytogenes strains involved in a multistate listeriosis outbreak associated with cantaloupe in US. PloS one. doi:10.1371/journal.pone.0042448Free full textPubMed 22860127DOI
Summary written 30 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error