Page 856 - Veterinary Immunology, 10th Edition

Moraxella bovis also secretes a leukotoxin for bovine neutrophils.
VetBooks.ir Actinobacillus pleuropneumoniae secretes a toxin that kills porcine

macrophages. Mycoplasma mycoides can kill bovine T cells. Other
bacteria trigger lymphocyte death by apoptosis. These include B.

anthracis, Streptococci, Shigella, L. monocytogenes, S. aureus, and
Yersinia. Leukotoxins are a relatively crude method of
assassination. Some bacteria use much more sophisticated methods
such as injecting toxins into their targets. Gram-negative bacteria

such as Salmonella, Pseudomonas, and E. coli have developed an
elaborate needle complex, a type III secretion system, to convey
effector molecules directly into the cytosol of effector cells. These
injection systems are turned on when a bacterium is ingested by a

cell and exposed to a low pH within the phagosome. Once the
needle complex enters the cytosol and detects its neutral pH,
injection of effector molecules occurs. These molecules activate
guanosine triphosphatases and disrupt intracellular signaling

pathways. At high concentrations, they produce transmembrane
pores and kill the cells.
Although killing leukocytes is an effective way to avoid being
eaten, other bacteria are content to simply prevent intracellular

destruction. Some bacteria generate a resistant cell wall to protect
themselves against lysosomal enzymes. For example, the cell wall
waxes of Corynebacterium pseudotuberculosis make it resistant to
lysosomal enzymes. S. aureus uses a cell wall peptidoglycan that is

completely resistant to lysozyme. Some bacteria produce
antioxidants that neutralize the products of the respiratory burst.
For example, the carotenoid pigments responsible for the yellow
color of S. aureus can quench singlet oxygen. Salmonella enterica

Typhimurium can prevent assembly of the NADPH-oxidase
complex and down-regulate host NOS2 activity. Pasteurella
multocida and Histophilus somni are also able to inhibit the
respiratory burst. Anthrax toxins LF and EF also inhibit NAPDH

oxidase activity. Staphylococcus aureus produces catalase that
inactivates hydrogen peroxide and the free radicals produced
during the respiratory burst. It also produces lactate dehydrogenase
that also helps it to resist NO-mediated oxidation. Some bacteria
can defend themselves against hypochlorite through the actions of a

molecular chaperone called Hsp33. This protein unfolds on

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