How does one isolate the microorganism that causes botulism from a steak? How is Staphylococcus Aureus isolated from tap water?

A toxin produced by the microorganism Clostridium
botulinum causes botulism.  This organism is present worldwide in soil and water.  Types
of botulism are as follows:

Food borne botulism:  Caused by
ingestion of uncooked, improperly canned foods, especially vegetables, cured pork, ham,
smoked or raw fish, honey and corn syrup.  Spoors of the organism in the food produce a
potent neurotoxin.  Symptoms occur 8-36 hours after ingestion and may include trouble
talking or swallowing, double vision, dry mouth, nausea and vomiting and trouble
breathing.

Wound botulism:  Occurs when organisms
contaminate an open wound, especially if the wound has dead tissue.  The spoors grow and
produce toxin in the wound.

Infant botulism:  Occurs when
the infant ingests the organism that contaminates certain foods, most notably honey and
corn syrup.  Infants under 1 year of age should never be given honey.  The organisms
grow in the infant’s gastrointestinal tract where the toxin is
produced.

To answer your question, the organism Clostridium
botulinum is not normally cultured or grown from the suspected food such as a steak. 
Instead, the laboratory detects the toxin itself.  The botulism organism can be cultured
or grown from an infected wound in wound botulism, and from the stool in the case of
infant botulism.

Botulism toxin is detectable in suspected
food and in the serum and stool of patients affected with food borne botulism.  The
toxin is also detectable (along with the organism) in the stool of patients with infant
botulism.

The second part of your question refers to
Staphylococcus aureus organisms in tap water.  This is not an issue, since staphylococci
are not ordinarily a contaminant of tap water.  Tap water may contain coliform bacteria
that are found in the intestines of man and animals, whose feces may contaminate water
supplies.  Tap water may also contain viruses and yeasts, and occasionally Legionella
bacteria, but not staphylococci.

Staph. Aureus becomes a
health issue when food is contaminated by contact with food preparers who may have a
staph. skin infection.  The organisms multiply in the food as it sits at room
temperature and produce a toxin.  The toxin, when ingested, causes cramps, diarrhea,
nausea and vomiting; so called staphylococcal food poisoning.