Editing Exotoxin (section)

=== Type II: membrane damaging ===

Membrane-damaging toxins exhibit [[hemolysin]] or cytolysin activity ''in vitro''.  However, induction of cell lysis may not be the primary function of the toxins during infection.  At low concentrations of toxin, more subtle effects such as modulation of host cell signal transduction may be observed in the absence of cell lysis. Membrane-damaging toxins can be divided into two categories, the channel-forming toxins and toxins that function as enzymes that act on the membrane.

==== Channel-forming toxins ====

Most [[Pore-forming toxins|channel-forming toxins]], which form pores in the target cell membrane, can be classified into two families: the cholesterol-dependent toxins and the RTX toxins.

* '''Cholesterol-dependent cytolysins'''

Formation of pores by [[cholesterol-dependent cytolysin]]s (CDC) requires the presence of [[cholesterol]] in the target cell.  The size of the pores formed by members of this family is extremely large: 25–30&nbsp;nm in diameter. All CDCs are secreted by the type II [[secretion]] system;<ref name="pmid16177291">{{cite journal | vauthors = Tweten RK | title = Cholesterol-dependent cytolysins, a family of versatile pore-forming toxins | journal = Infection and Immunity | volume = 73 | issue = 10 | pages = 6199–209 | date = October 2005 | pmid = 16177291 | pmc = 1230961 | doi = 10.1128/IAI.73.10.6199-6209.2005 }}</ref> the exception is [[pneumolysin]], which is released from the cytoplasm of ''[[Streptococcus pneumoniae]]'' when the bacteria lyse.

The CDCs ''Streptococcus pneumoniae'' Pneumolysin, ''[[Clostridium perfringens]]'' [[perfringolysin O]], and ''[[Listeria monocytogenes]]'' [[listeriolysin O]] cause specific modifications of [[histone]]s in the host [[cell nucleus]], resulting in down-regulation of several genes that encode proteins involved in the [[inflammatory response]].<ref name="pmid17675409">{{cite journal | vauthors = Hamon MA, Batsché E, Régnault B, Tham TN, Seveau S, Muchardt C, Cossart P | title = Histone modifications induced by a family of bacterial toxins | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 33 | pages = 13467–72 | date = August 2007 | pmid = 17675409 | pmc = 1948930 | doi = 10.1073/pnas.0702729104 | bibcode = 2007PNAS..10413467H | doi-access = free }}</ref> Histone modification does not involve the pore-forming activity of the CDCs.

* '''RTX toxins'''

[[RTX toxin]]s can be identified by the presence of a specific tandemly repeated nine-amino acid residue sequence in the protein.  The prototype member of the RTX toxin family is [[haemolysin A]] (HlyA) of ''E. coli''.{{Citation needed|date=March 2009}} RTX is also found in ''[[Legionella pneumophila]]''.<ref name="pmid18194518">{{cite journal | vauthors = D'Auria G, Jiménez N, Peris-Bondia F, Pelaz C, Latorre A, Moya A | title = Virulence factor rtx in Legionella pneumophila, evidence suggesting it is a modular multifunctional protein | journal = BMC Genomics | volume = 9 | pages = 14 | date = January 2008 | pmid = 18194518 | pmc = 2257941 | doi = 10.1186/1471-2164-9-14 | url =  | doi-access = free }}</ref>

==== Enzymatically active toxins ====

One example is the [[Clostridium perfringens alpha toxin|α toxin]] of [[Clostridium perfringens|''C. perfringens'']], which causes [[gas gangrene]]; α toxin has [[phospholipase]] activity.