Simplistically speaking, antibiotic-responsive diarrhea is a case of diarrhea that responds to antibiotic therapy. There are
several different terms that describe similar clinical conditions: antibiotic-responsive diarrhea, tylosin-responsive diarrhea,
small intestinal bacterial overgrowth (SIBO), and intestinal dysbiosis. At the current time it is unclear, whether all 4 terms
describe essentially the same condition or if only one of these terms would be most appropriate.
As said above, antibiotic-responsive diarrhea is a case of diarrhea that responds to antibiotic therapy. As such, tylosin-responsive
diarrhea would be similar. However, this term was coined by a Finish group after they had done several studies in dogs with
chronic diarrhea. The dogs showed poor response to several different antibiotics, but all responded to tylosin. The reasons
for these findings are unclear. One explanation is that tylosin has an optimal antibiotic spectrum against the intestinal
bacteria that are responsible for the diarrhea. Another explanation is that tylosin has other properties in addition to its
antibiotic properties. Small intestinal bacterial overgrowth refers to an expansion of unfavorable bacteria in the small intestinal
tract. Finally, dysbiosis refers to a qualitative and/or quantitative derangement of the small intestinal microbiota that
leads to clinical signs of small bowel diarrhea. Again, it is unclear whether these conditions can really be separated and
for a lack of better understanding in the following text the term antibiotic-responsive diarrhea is used as an overarching
term for all four conditions.
The microbiota
The intestinal microbiota is made of a wide variety of microorganisms, including bacteria, viruses, and fungal organisms.
Most attention has been given to the intestinal bacterial ecosystem, which is made up of a complex mixture of a wide variety
of bacterial species. Traditional studies describing the intestinal bacterial ecosystem have employed traditional culture
techniques. Unfortunately, such studies are associated with problems in reproducibility. For example, a variety of studies
reported the physiologic bacterial ecosystem in the proximal small intestine of dogs, but different studies found a preponderance
of different bacterial species. It has since been recognized that a variety of factors, such as location, breed, age, collection
method, culture media, culture conditions, and others all play an important role in the results of culture-based studies.
However, the true diversity of the intestinal bacterial ecosystem became evident only recently with the advent of new micromolecular
technologies. These newer technologies have revealed a far greater diversity of the bacterial ecosystem in the intestinal
tract than previously assumed and have also shown that fungal organisms, such as Pichia spp., Cryptococcus spp., Candida spp., and Trichosporon spp. are far more frequently present in the intestinal tract of healthy dogs than previously believed. Using these new methodologies
it has now been estimated that the intestinal bacterial ecosystem is made up of more than 1000 different bacterial species.
These new studies are also crucial in studying the microbiota in patients gastrointestinal diseases.
Physiologic importance of the intestinal bacterial ecosystem
This ecosystem is initially established during birth and continues to develop during suckling. The impact of the intestinal
microbiota and the bacterial ecosystem has been well established by studies in germ-free rodents. These rodents show a wide
variety of morphological and physiological alterations that overall equate to a state of compromised intestinal function and
immunity. In healthy animals the physiologic microbiota, and most prominently the bacterial ecosystem, has several important
functions. Firstly, it protects the host against pathogenic bacteria, by competing for oxygen, luminal substrates, and space,
but also by synthesizing and releasing substances that inhibit bacterial growth, so-called bacteriocins. Intestinal bacteria
also produce short-chain fatty acids by metabolizing dietary components that are often non-digestible for the host. In turn,
these short chain fatty acids serve as an important energy source for the intestinal mucosa, leading to epithelial cell proliferation
and mucosal growth. Members of the intestinal bacterial ecosystem also synthesize a variety of vitamins, including riboflavin
(vitamin B2), biotin (vitamin B7), folic acid (vitamin B9), cobalamin (vitamin B12), and vitamin K. It is important to note,
however, that physiologically, the synthesis of some of these vitamins, for example cobalamin, is not of any significance
to the host as the synthesis may occur distally to where the vitamin can be absorbed. Finally, intestinal bacteria also play
a crucial role in the development of the intestinal immune system. They stimulate said intestinal immune system, which plays
a crucial role in overall host defense throughout all stages of life.
