I would prefer this question be answered by a medical professional, if
possible, due to the "personal experience" factor of the question...

Assume a patient you are treating has some type of systemic bacterial
infection. You review the many symptoms with the patient, but are
unable to narrow the list to even 10 types of bacteria, based upon the
symptoms. How would you methodically arrive at a diagnosis? Is there
any type of all-encompassing blood and/or fluid bacterial composition
test that can be performed, describing the bacteria present in given
blood/fluid?

Hello -

Systemic bacterial infection, bacteremia (the presence of viable
bacteria in the blood), and the associated inflammatory response to
such infection, sepsis, have similar symptoms regardless of the
underlying infectious cause - fever, chills, rigor (shaking),
increased/decreased white blood cell levels,  increased heart rate,
and increased respiratory rate are among the most common symptoms
seen.  Thus, the symptoms that the patient may exhibit provide little
diagnostic information as to the casual bacteria (although there is
definitely a short list of the most typical bugs).  Often times, the
first step in determining the offending organism is to attempt to
identify the source of the infection - systemic bacterial infections
most often start in a single organ and by overwhelming the bodies
defences are able to spread into the bloodstream and to other organs. 
Depending on the organ of origin (ie lung, urinary tract, bowel,
abscess), certain bacteria are more likely to be the cause of the
infection.  Treatment may be given at this point, with only a
suspected origin, and thus most likely bacteria, if the patient's
condition is critical enough.

Samples of fluids from patients with an infection may be examined
under a microscope to provide some information regarding the nature of
the infectious source - bacteria can sometimes be seen if the levels
are high enough, and some characteristics can be determined (most
common are gram staining and acid-fast staining - and the shapes and
type of growth of the bacteria seen) - this can allow the
identification of bacteria within the fluid - a staphylococcus aureus
infection (gram positive, cocci in clumps), a mycobacteria
tuberculosis infection (acid fast positive), e. coli infection (gram
negative rods).  Not all bacteria are visible (eg listeria) in this
manner, and a negative result does not disprove the existence of
bacteria in the sample.

The best, but also the slowest, method for establishing the microbial
origin of an infection is to culture the suspect fluid.  Cultures are
performed on many types of growth media to segregate bacteria by
nutritional (presence of certain factors or nutrients) and
environmental requirements (aerobic vs anaerobic).  Cultures may give
positive results in as little as a day if the infectious source is at
high levels in the sample fluid and is capable of growing quickly;
however, some bacteria only grow slowly and may not be seen for many
days (campylobacter for instance).  Cultures not only allow for the
determination of the organisms present in the fluid, but, once an
organism is identified, it can be grown on other growth media
impregnated with antimicrobial agents to test their effect upon the
organisms for a particular infection ("sensitivity testing" - this is
how "resistant" infections are determined - the bacteria grow in
culture even in the presence of the drug, such as penicillin, to which
they are resistant).  As culturing the organisms takes time, treatment
may be begun shortly after drawing the samples for culture based on
expectationso of likely infectious causes - the treatment can then be
more specifically tailored once the results from a culture are
avaialble (and may be further tailored by sensitivity testing).

Not all organisms are easily identified in this manner as not all
bacteria can be cultured easily, or at all.  In this case, other tests
are available based upon using probes to particular DNA, RNA or
protein components of the bacteria to establish their presence in a
sample of interest.  These tests are usually quite quick, but the
correct test must be chosen for the organism of interest - thus these
tests are usually only used when one has suspicion of a particular
infection (ie a chlamydia test is usually only run if one believes
that the infection could result from chlamydia).

Good references:

A nice overview of the topic:
http://www.chclibrary.org/micromed/00039230.html

A general overview of bacteremia and sepsis:
http://www.nlm.nih.gov/medlineplus/ency/article/000666.htm

The Merck manual section on bacteremia:
http://www.merck.com/pubs/mmanual/section13/chapter156/156b.htm



Please let me know if you have further questions.

synarchy


Search strategy:
professional knowledge used for answer
references found by Google searches, terms: bacteremia, sepsis,
diagnosis

Perfect. Beyond my expectations. Thanks synarchy.