[rodesh (OP)]

Why are some bacterial strains within the same serovar and definite phage type responsible for causing outbreaks while others are not? Could it be just due to chance or are there genetic differences that could result in certain strains being associated to outbreaks?

[evan_au]

By "outbreak", I assume you mean "causing humans to get sick and die"?

In the case of cholera, what makes it deadly is the toxin it produces, which triggers the digestive system to try and flush it out, often with lethal consequences.
- Most strains of cholera don't produce this toxin, and could live safely in the human gut. Only 0.1% produce the toxin.
- If one of the strains that normally do produce the toxin had a mutation which deactivated the toxin, then this strain would be classified by serotype with the deadly strain, but would not cause a disease outbreak.
- Similarly, if one of the strains that normally do not produce the toxin gained a gene for the toxin (eg by horizontal gene transfer), then this strain would be classified by serotype with the "safe" strains, but would cause a disease outbreak.

Serovar (and bacteriophage binding) are mostly about the surface proteins.
- But other genes control the behavior of the cell and proteins within the cell, and may not have such an impact on the surface proteins

See: https://en.wikipedia.org/wiki/Serotype

[rodesh (OP)]

Hi! Sorry I was not very clear. What I meant by "outbreak" is when there is an occurrence of the disease in an area at a rate that is higher than normal. So why can some strains cause an outbreak event to occur within a population whereas others don't?

[evan_au]

So why can some strains cause an outbreak event to occur

There are many reasons...
Contagious Pathogens
A characteristic of contagious pathogens that has been brought to public attention by the current COVID outbreak is the "R₀" value. This is the number of people who would be infected by 1 person, if no special precautions are taken.

If R₀ < 1, the outbreak will die out.  Eg if R₀=0.5, and 10 people get infected initially, the number of infected people will shrink over time:10, 5 , 2, 1... 0

If R₀>1, the outbreak will grow. Eg if R₀=2, and 10 people get infected initially, the number of infected people will grow over time: 10, 20, 40, 80, 160....  Obviously, this can't go on forever - it will be limited by the human population. But it will slow down earlier as the number of immune contacts rises.
- Factors that can increase R₀ are:
- If it is spread through the air by coughing and sneezing (especially if the pathogen promotes coughing and sneezing)
- If it is spread through poor water supply (especially if the pathogen promotes diarrhea)
- If the pathogen becomes contagious before infected patients start showing symptoms

The goal of "flattening the curve" is to take steps like social distancing and contact tracing, so that fewer people are infected, bring R down to < 1.
See: https://en.wikipedia.org/wiki/Basic_reproduction_number

Non-Contagious Pathogens
In this case, the pathogen does not spread directly from person to person, but comes from an reservoir. Eg salmonella is endemic in chickens, so eating raw eggs, or leaving uncooked chicken at room temperature for an extended period can produce localized outbreaks.

In some cases, a pathogen grows on vegetables, and these can multiply and spread in packing factories, to be shipped all over the country. Rigorous cleaning of production facilities, and using clean sources of water will help in these cases.

In some cases, a pathogen lives in animals, and sometimes jumps into humans.

Hybrid Outbreaks
Ebola and Coronaviruses originate in wild animals, and sometimes jump into humans, where it can spread from human to human.

Sometimes the source of food-related outbreaks is improperly treated human sewage.
 
In some cases like Malaria and other tropical diseases, an insect vector carries the disease from person to person.

Non-Pathogen
Most microbes just don't care for humans - they may prefer fruit, or dead leaves on the forest floor, or pigs.
Some microbes are even beneficial to humans, and live in us or on us, as part of our microbiome.
These don't cause disease outbreaks, but they can still affect individuals with compromised immune systems, eg the aged, transplant patients, HIV patients or chemotherapy patients.

[rodesh (OP)]

Excellent! Thank you very much for the input. This was exactly what I was looking for. Once again, thank you!

[joemartin]

great input