Attila, you are coming to what I see as a language problem regarding how we think about our microbial ecosystem.  Infection is the 'older' word used which was quite understandable in the past with our limited understanding of our microbiota, even infections may generally be polymicrobial but instead we focus on one recognizable strain to determine our therapy.  Campylobacter may be a commensal at low (undetectable) levels early in life, but during the stressful times it overgrows and is shed because of its high numbers.  The better term for this is 'dysbiosis' which means a microbial imbalance, and implying that it is not a good situation even if we do not see a strong pathological impact.  We need to use this term more often for these imbalances and hope it catches on as it is probably more descriptive and correct than the usual meaning of infection.

Interesting question. Is right what you think, bacteria may be normal or pathological according to many factors, including the location in the body, virulence, quantity, the host and on the other hand the host immune system. Example: Clostridium difficile is normal flora of the intestine but an imbalance in the normal gut flora can cause pseudomembranous colitis. Staphylococcus aureus is skin flora but can cause serious infections wounds. Escherichia coli is normal flora in intestine but the bacteria are específicosa serotypes are enteropathogens. Campylobacter is usually normal flora in feces of birds but is pathogenic to humans.

Best regards

I agree with the basics of the question and answer from Marielsa.  But the word commensal often indicates an established relationship that is ongoing.  Sometimes it might be better to consider some bacteria to be colonizers, or say that they can colonize a part of the body for a period of time.  Staphylococcus aureus (Sa) is often considered to be neither a part of normal skin flora (not a commensal, or mutualism relationship), but an organism that is able to infect when given the opportunity.  Many researchers use the word colonizer for Sa as it can be present and then apparently not be present for periods of time.  Sa may actually be a commensal if it is generally found somewhere, or in numbers too low to detect, but would be a colonizer if it is normally not present on the skin or in the nares except for certain periods of time/location.  The problem of course is that it will become a pathogen when it can grow and cause some form of harm to the body, and it has proven to quite capable of doing that. 

I also agree with the above. But for example in broilers when they hatch they are not considered "infected". Then they are reared on a farm and then they are sampled after lets say thinning, which is stressful for the birds and they shed Campylobacter in the faeces. From this point onwards and even at the arrival to the slaughterhouse the flock is considered "infected". How, why and when does the state of the flock changes from being clear (no Campylobacter detected) to being infected? So my question is: is this a misuse of the term "infection" when Campylobacter is "present", but it is not established whether it is actually infecting the birds or not?

Attila, you are coming to what I see as a language problem regarding how we think about our microbial ecosystem.  Infection is the 'older' word used which was quite understandable in the past with our limited understanding of our microbiota, even infections may generally be polymicrobial but instead we focus on one recognizable strain to determine our therapy.  Campylobacter may be a commensal at low (undetectable) levels early in life, but during the stressful times it overgrows and is shed because of its high numbers.  The better term for this is 'dysbiosis' which means a microbial imbalance, and implying that it is not a good situation even if we do not see a strong pathological impact.  We need to use this term more often for these imbalances and hope it catches on as it is probably more descriptive and correct than the usual meaning of infection.

Robert, this is the problem I try to understand regarding the "language". I found that depending on the field of study (microbiology, poultry science, meat science, food processing) there are terms that are used in general to describe something (like infection), however scientist on different fields might understand something different on that same term (infection). I notice this often as I am quite interdisciplinary in my research.

Attila, you are exactly correct.  Each field uses the terms in their own ways, based on their fields' history of usage, and often failing to recognize (or explain) how their terminology affects how their research is understood by others.  I would encourage you to always define your terms, even when they should be understood, and encourage the terminology to move toward a more universal understanding.  I too am very interdisciplinary and the terminology can cause many problems.  I have posted here on RG about the problem of researchers using only the word 'spore' when they specifically mean 'endospore'  and how that creates some misunderstanding (there are so many types of spores, but also it creates a huge problem in our new computer age (how can I find all the research on endospores if some researchers never used this term in their papers, even though it was about endospores).(https://www.researchgate.net/post/Should_the_term_Endospore_always_be_used_preferentially_to_spore 

I do wish 'we' (all researchers) could come to a better understanding (of the concepts of infection, commensalism, colonizing, and dysbiosis) and then use these terms more appropriately to help move us all forward and make searching for answers easier. 

Once a microorganism breaks intestinal or other cellular barriers, it is an infection. This does not mean infectious diseases. Probably, far less than 0,1% of all infections will lead to any clinically overt or even measurable symptoms. The ability of the bug to break barriers is strictly dependent on the species. For example, like campylobacter, also salmonella are shed in high concentrations by a number of birds without causing any symptoms. Once taken up by man, they can break barriers and lead to symptoms.

The wording commensal is traditional and a bit outdated and means that you can find the bug in a given compartment of most individuals of the species (i.e. poultry) and therefore the bug is part of the normal microbiota in this species.

Microbiota can be a commensal in the natural environments e.g.,Escherichia coli..Helicobacter pylori(formerly Campylobacter) a pathogen in humans.For more information you may read my two articles1].Risk Factors,Diag--of Peptic Ulcer Disease.Doi:109790/0853-14784046.[2]SEroplavalence and risk factors for H.pylori--in East Malaysian Community.Int J Med Pharm Sci.2013;3(5):49-56..Regards

Dr. Solbach's comments are well taken, and his definition of infection is the standard definition.  Some definitions appear to allow that microorganisms that enter the body and grow are also causing an infection.  The biggest problem that I have with these definitions is that it does not account for situations where microorganisms live in a body structure, do NOT penetrate a cellular or even membrane barrier and yet they produce a variety of toxins that do cause harm.  Are these exotoxins then a part of an infection or are they simply the overgrowth of a 'colonizing' pathogen?  Examples of exotoxin producing 'infections' that cause harm include some 'food poisonings,' many UTI's, and even 'sore throats' where hemolysins diffuse into the blood.  So does the definition of infection mean not only when a microorganism crosses a 'barrier' but also if a product (eg. exotoxin) is the only 'part' of the organism to cross a 'barrier'?  Also, if skin is damaged by an abrasion and bacteria grow in the surface secretions, is this a 'soft tissue infection' or only a 'soft tissue colonization'?  We do have some definitional questions that need to be resolved, or we need to always add our definition to the terms we use. 

I also accept the comments of Dr. Solbach on the definition of infection Further, as he mentioned Campylobacter and Salmonella can be shed in high concentrations by birds without causing any harm in the bird. So, why are these birds called infected then in many articles and in the poultry industry? Most of the time it is not known whether the birds are actually infected.

While the definition of Dr. Solbach is important, damage from toxins is also important.  And just because we may not see damage clearly, the presence (and shedding) of Campylobacter and Salmonella may cause inapparent harm.  It may be as simple as slower weight gain, but just because harm may not be noticeable does not mean that there is none.  There is often more going on than just what is easily detectable.  Never completely trust our limited abilities to know everything going on in complex systems as our 'tools' have only limited information which they can provide.