In contrast, other published studies have often been selective in terms of the types of travelers, geographic areas visited, or attempts to isolate specific microorganisms with the aim of testing antibiotic efficiency.
Since, in natural environments, these parameters are highly dynamic, microorganisms have to cope with fluctuating, often non-optimal growth conditions.
Most often, these microorganisms have been studied for a number of practical reasons relative to either a positive or a negative impact on humans and economy.
More often, many different microorganisms have been associated with a single autoimmune disease, which indicates that more than one infectious agent can induce the same disease through similar mechanisms (Table) (9 ).
Iron is often limited during infection and microorganisms have developed strategies, such as the production of siderophores, to maximize iron acquisition.
And scientists who study microorganisms have pointed out that regularities regarding reproduction of macroorganisms often have little or no applicability in the world of the very small.
Some microorganisms have become resistant to drugs, requiring a continuing search for different (and often more expensive) agents.
Many microorganisms have genes coding for pathways of secondary metabolism and these pathways are often turned on only under a specific set of stress conditions.
Lastly, this approach provides no information on the absolute mRNA abundance, which is often useful since it has been established in that global transcriptional levels in microorganisms have strong correlation with global protein levels and gene essentiality [ 4- 6].
About 6,000 species of microorganisms have been formally described.
