Such death curves can be empirically established for all bactericidal agents. This D-value is affected by pH of the product where low pH has faster D values on various foods. The D-value at an unknown temperature can be calculated knowing the D-value at a given temperature provided the Z-value is known.
The target of reduction in canning is the D reduction of Clostridium botulinum , which means that processing time will reduce the amount of this bacteria by 10 12 bacteria per gram or milliliter.
A D reduction will take seconds. Different microbial structures and types of microbial cells have different level of resistance to antimicrobial agents. Different microbial structures and types of microbial cells have different level of resistance to antimicrobial agents used to eliminate them.
Endospores are considered the most resistant structure of microbes. They are resistant to most agents that would normally kill the vegetative cells from which they formed. Nearly all household cleaning products, alcohols, quaternary ammonium compounds and detergents have little effect.
However, alkylating agents e. Prolonged exposure to ionizing radiation, such as x-rays and gamma rays, will also kill most endospores.
Bacillus subtilis stained with the Schaeffer-Fulton stain. Certain bacterial species are more resistant to treatment than others. Mycobacterial infections are notoriously difficult to treat. The organisms are hardy due to their cell wall, which is neither truly Gram negative nor positive. In addition, they are naturally resistant to a number of antibiotics that disrupt cell-wall biosynthesis, such as penicillin.
Due to their unique cell wall, they can survive long exposure to acids, alkalis, detergents, oxidative bursts, lysis by complement, and many antibiotics. Most mycobacteria are susceptible to the antibiotics clarithromycin and rifamycin, but antibiotic-resistant strains have emerged. Protozoa cysts are quite hard to eliminate too. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time.
Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. Protozoa cells are also hardy to eliminate. Gram-negative bacteria have high natural resistance to some antibiotics. Examples include Pseudomonas spp.
This ability to thrive in harsh conditions is a result of their hardy cell wall that contains porins. Their resistance to most antibiotics is attributed to efflux pumps, which pump out some antibiotics before the antibiotics are able to act. Staphylococcus aureus is one of the major resistant pathogens. Found on the mucous membranes and the human skin of around a third of the population, it is extremely adaptable to antibiotic pressure. It was one of the earlier bacteria in which penicillin resistance was found—in , just four years after the drug started being mass-produced.
A recent study demonstrated that the extent of horizontal gene transfer among Staphylococcus is much greater than previously expected—and encompasses genes with functions beyond antibiotic resistance and virulence, and beyond genes residing within the mobile genetic elements. Fungal cells as well as spores are more susceptible to treatments. Vegetative bacterial and yeasts cells are some of the easiest to eliminate with numerous agents and methods.
Bacterial endospore is the hardest organism to be killed by disinfectant. During stress condition, some of the bacteria will undergo sporulation and form spore coat to protect themselves from dying. The endospore remains dormant until the environment is favourable for it to grow.
Bacillus and Clostridium have been widely used as the model to study the properties of bacterial endospore. Bacterial endospore can be permanently deactivated by disinfectant containing chloride compounds, glutaraldehyde or peracetic acid.
In contrast, alcohol, phenolics, QACs and chlorhexidine are not effective in eliminating bacterial endospore 3. Mycobacteria is the second hardest organism to be eliminated by disinfectant. This might due to their complex cell walls that provide effective barrier against the active substance.
Phenol, peracetic acid, hydrogen peroxide, alcohol and glutaraldehyde are active substances that exhibit mycobactericidal activity. In contrast, chlorhexidine, glutaraldehyde and QACs are weak in mycobactericidal activity. Small non-enveloped viruses, including norovirus and poliovirus, are the most resistant virus group against disinfectant. The size for small non-enveloped virus is less than 50nm. This virus group contains a very resistant viral capsid. Gram-negative bacteria contains cell wall which is composed of lipopolysaccharide and proteins.
This type of cell wall limits the entry of antibacterial agents to the cell, and thus gram-negative bacteria is considerably more resistant to most of the active substances of disinfectant, including chlorhexidine and QACs, compared to gram-positive bacteria. Fungi can be divided into yeast and mold. Yeast reproduce asexually through mitosis, while mold reproduce through spores.
The mechanisms involved in biocide resistance to biocides are becoming better understood. Intrinsic resistance intrinsic insusceptibility is found with bacterial spores, mycobacteria and Gram-negative bacteria.
This resistance might, in some instances, be associated with constitutive degradative enzymes but in reality is more closely linked to cellular impermeability. The coats s and, to some extent, the cortex in spores, the arabinogalactan and possibly other components of the mycobacterial cell wall and the outer membrane of Gram-negative bacteria limit the concentration of active biocide that can reach the target site s in these bacterial cells.
A special situation is found with bacteria present in biofilms, which can be considered as being an intrinsic resistance mechanism resulting from physiological phenotypic adaptation of cells.
Acquired resistance to biocides may arise by cellular mutation or by the acquisition of genetic elements.
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