Type II (pore forming ) toxins(e.g., alpha-toxin S aureus and listeriolysin O from L monocytogenes) destroy the integrity of eukaryiotic cell membran There are two types membrane-diisrupting toxin a protein that forms channels in the membrane because of differences in oscomotic strength between able host cell cytoplasm and the environment, these holes in the membrane trigger rush of water into the cell, rupturing the cell and an enzyme (phospholipase) that degrades phospholipids in the membranes, removing the polar head group of the phospholipid.

Type III toxins, or A-B toxins, are composed of two types subunits : a”B” subunit, which recogainez and bins to the host cell receptor (usually a carbohydrate moiety) and an “A” subunit , which has azymatic activity A-B toxins can be simple , with only one A and one B subunit, or they can be composed of several B subunits and only one A subunit . The A and B subunits are usually separated by a proteolytic cleaving event, and they remain connected through a disulfide bond. Both the simple anf the compound toxins bind to enter the host cell. The B subunit binds to the host receptor ( which is what determines the host cell specifity of the toxin). Following this binding the toxin is endocytosed by the cell and the A subunit (active enzymatic subunit) is translocated to the cytoplasm (Fig. B-13). The A subunit of different toxins eneter different cell types according to the distribution pf the receptors for the B subunits (which confer hostcell specificity). Homever, most A subunits catalyze the same reaction: they remove the ADP-ribosyl group frpm NAD and attach it covalenty to a hostcell protein.

The effect of this reaction vary according to the host cell protein that has been ADP ribosylated. The A subunit of diphtheria toxin ADP-ribosylated elongation factor 2(Ef-2) a protein that plays an essential role in host cell protein synthesis . The a subunit of cholera toxin ADP –ribosylates a regulatory enzyme that controls cylic AMP levels in the host cell : this prevents the enzyme from being turned off , causing the host cell to lose control of ion flow and resulting in a massive loss of host cell water , which is seen diarrhea . other a subunits have different activities .Shiga toxin A subunit cleaves a host cell rRNA molecule ,preventing protein synthesis . Table B-3 depicts a summary of the comparable propertis of exotoxins and endotoxins