B

babesiosis An illness that results fromINFECTION
with the parasitic protozoan Babesia microti. Most
people who have babesiosis do not have symp-
toms; the infection causes illness primarily in peo-
ple who are IMMUNOCOMPROMISEDor who have had
SPLENECTOMY(surgical removal of the SPLEEN). The
bite of the Ixodestick, found in the northeastern
United States, is the mode of transmission.
Babesiosis is rare in other parts of the United
States.
B. microtiinfects the erythrocytes (red BLOOD
cells), causing alterations in their cell membranes
that affect their ability to carry oxygen. Hemolytic
ANEMIAis a key consequence of babesiosis. Symp-
toms may include FEVER, COUGH, and shortness of
breath (DYSPNEA). The doctor uses blood tests to
diagnose babesiosis. The tests show the damage to
the erythrocytes and the presence of antibodies.
Treatment with ANTIBIOTIC MEDICATIONScures the
infection. Rarely, a person may develop the life-
threatening complication ACUTE RESPIRATORY DIS-
TRESS SYNDROME(ARDS).

ANTIBIOTIC MEDICATIONS TO TREAT BABESIOSIS

atovaquone azithromycin
clindamycin quinine sulfate

See alsoANTIBODY; ERYTHROCYTE; GIARDIASIS; LYME
DISEASE; PROTOZOA.

bacteria Single-cell microorganisms (microbes).
Bacteria are the most ancient and primitive life
forms known, with fossils dating back more than 3
billion years. A bacterium’s structure is very sim-
ple, consisting of a rigid cell wall that supports and
contains the cytoplasm, fragments of RNA, and a
single strand of DNA within a nonencapsulated
(unbordered) nucleus. Though bacteria are capa-

ble of independent existence, most require a sym-

biotic relationship with a host organism. The bac-

teria provide needed functions for the host in

exchange for NUTRIENTSand safe haven.

Many types of bacteria exist in and on the body

in just such a symbiotic partnership; these are part

of the body’sNORMAL FLORA. Bacteria in the gas-

trointestinal tract digest food, for example. Bifi-

dobacterium bifidum, Lactobacillus acidophilus, and

Saccaromyces boulardiiare some of the more abun-

dant bacterial families that reside in the small

intestine. However, when normal flora bacteria

are able to establish themselves in tissues other

than their natural habitat or their numbers

become abundant, they cause INFECTION. Escherichia

coli, for example, are abundant normal flora in the

COLON, where they work to prepare the residue of

digestion for elimination from the body. E. coli also

synthesize VITAMINK, which is essential for COAGU-

LATION(BLOODclotting). When E. coliescape from

their habitat, however, they cause infections such

as VAGINITISor URINARY TRACT INFECTION(UTI).

THE “BAD” E. COLI:O157:H7

The bacterial family Escherichia coliis extensive

and ubiquitous—its many strains live in the gas-

trointestinal systems of nearly all animals. E. coli

O157:H7,NORMAL FLORAin cattle, is a family

member of great notoriety for the potential of

severe illness it presents in people. The toxin this

strain releases can destroy red BLOODcells in

such volume that the KIDNEYSfail, a condition

called HEMOLYTIC UREMIC SYNDROME. E. coli

O157:H7 enters the human food chain as a

foodborne illness.

Bacteria that cause infection are pathogens.

Most pathogenic bacteria exist in the natural envi-

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