GENERAL | RESISTANCE | SURVIVAL Resistance to heat and cold Resistance to ultraviolet light (UV) and gamma irradiation Resistance to chemicals: antibiotics and disinfectant		Mycobacteria are notoriously resistant to physical and chemical factors but M. paratuberculosis seems to be among the most resistant of mycobacteria.

Resistance to heat & cold

The thermal tolerance of M. paratuberculosis, specifically the capacity to survive pasteurization, is the subject of considerable interest. Some published reports suggest that M. paratuberculosis can survive standard commercial pasteurization while others suggest it can not. Thermal tolerance curves indicate that M. paratuberculosis is comparable in heat resistance to M. avium and far more heat resistant than Listeria, another facultative intracellular bacterium that is found in raw milk.

Concerning cold, Richards and Thoen showed that the number of living organisms in fecal samples from cattle naturally and experimentally infected with M. paratuberculosis was significantly decreased after freezing at -70°C after 3 weeks. Continued refrigeration up to 15 weeks did not result in further decline in the number of M. paratuberculosis.

Studies on suspensions of M. paratuberculosis (10⁶/ml) in broth culture media held at refrigerator temperatures (4°C) gave similar results. Counts of living bacteria declined precipitously with greater than 1 log decrease within 5 days. Then, a residual population of apparently cold-tolerant M. paratuberculosis cells (roughly 1% of the starting number) persisted to the end of the experiment at 25 days (Collins, unpublished data). For comparison, in studies on the cause of bovine tuberculosis (Mycobacterium bovis) when 10⁷ M. bovis/ml were suspended in phosphate buffer (pH 7.2) and held at refrigerator temperatures (2-4°C), 50% survival was found after 21 days and 2% survived after 1 year.

Resistance to ultraviolet light (UV) and gamma irradiation

UV doses required for bacterial and viral inactivation are relatively low, typically in the range of 2 to 6 mW-s/cm² for 1 log inactivation. (Since water characteristics such as pH, hardness, turbulence, turbidity, and biological oxygen demand dramatically affect UV disinfection efficiency, any generalization of these doses to other water treatment protocols would be ill advised.) When 10⁵-10⁶ M. paratuberculosis were suspended in sterile deionized water 4 mW-s/cm² was sufficient to achieve a 1 log reduction in viable counts and at UV doses greater than15 mW-s/cm² complete disinfection was achieved (Manning, unpublished data).

Older literature concerning the effects of natural sunlight on mycobacteria in the environment indicate that sunlight (presumably UV radiation) decreases the survival rate and that M. paratuberculosis is more resistant to adverse effects of sunlight than is M. bovis. Recent work in Australia, however, indicates that UV light had minimal effect on M. paratuberculosis viability in soil spiked with the bacteria.

A study on irradiation of M. paratuberculosis suspended in bovine colostrum found that if frozen bovine colostrum spiked with 10⁴ M. paratuberculosis/ml was exposed to 10 kGy (kilogray) gamma irradiation (60Co gamma-beam facility in Dagneux, France), 100% of the organisms were killed. To place this dosage of gamma-irradiation in perspective it should be noted that 7 kGy is the maximum allowable dosage for meat treatment in the U.S., 30 kGy is the allowable dose for treatment of dried spices, and 40 kGy is used to sterilize foods for the NASA (U.S.) space program.

Resistance to chemical factors: antibiotics and disinfectants

Mycobacteria are notorious for their resistance to antibiotics that kill most other bacteria. Only a select few antibiotics can be used to treat mycobacterial infections effectively and in most cases the course of therapy is weeks to months. M. paratuberculosis, like its close relative M. avium, is even resistant to antibiotics that normally are efficacious against M. tuberculosis, the cause of tuberculosis. Antimicrobial therapy for Johne's disease is not often attempted, as the cost of the drugs for these large animals and the duration of treatment required make it cost-prohibitive for livestock.

M. paratuberculosis, like other mycobacteria, are resistant to common disinfectants. However, phenolic and cresylic disinfectants are effective. Commercial disinfectant products labeled "tuberculocidal" should generally be effective against M. paratuberculosis. Research in this area was done in the 1950s and there is little current information to substantiate these observations or make more specific recommendations regarding products, concentrations or required contact times. 1-Stroke Environ® is a product commonly used by veterinarians that is believed to be effective at killing M. paratuberculosis.

M. avium is more resistant to free chlorine (bleach) than are most other bacteria. At a concentration of 1 mg/liter the time for a 1 log reduction in viable M. avium counts was roughly 50 minutes. By contrast, the time to achieve a 1 log reduction in E. coli was 28 sec. There was variation in chlorine susceptibility by the M. avium strain tested. Those strains that were slowest growing were most resistant. Water-grown M. avium cells were 10-fold more chlorine-resistant that culture medium grown cells.

These laboratory findings are supported by epidemiological studies. A survey of disinfection practices in the U.S. found that water utilities maintain a median chlorine residual of 1.1 mg/ml and a median exposure time of 45 min before the point of first use in the distribution system. Despite this practice, in Los Angeles, California, nontuberculous mycobacteria (M. avium and others) were isolated from water in 82% of 55 homes, and 100% of water in 31 commercial buildings, and 15 hospitals. Comparable studies for M. paratuberculosis have not been reported, however, unpublished findings indicate that M. paratuberculosis is as, if not more, chlorine-resistant than M. avium (Collins, unpublished data), an observation consistent with the reported relationship between growth rate and chlorine resistance.