A Comparison of Biological and some Cultural Methods for the Primary Isolation of Mycobacterium Tuberculosis

Diagnosis of mycobacteria in bovine milk: an overview

Tuberculosis remains as the world's biggest threat. In 2014, human tuberculosis ranked as a major infectious disease by the first time, overcoming HIV death rates. Bovine tuberculosis is a chronic disease of global distribution that affects animals and can be transmitted to humans by the consumption of raw milk, representing a serious public health concern. Despite the efforts of different countries to control and eradicate bovine tuberculosis, the high negative economic impact on meat and milk production chains remains, given the decreased production efficiency (approximately 25%), the high number of condemned carcasses, and increased animal culling rates. This scenario has motivated the establishment of official programs based on regulations and diagnostic procedures. Although Mycobacterium tuberculosis and Mycobacterium bovis are the major pathogenic species to humans and bovines, respectively, nontuberculous mycobacteria within the Mycobacterium genus have become increasingly important in recent decades due to human infections, including the ones that occur in immunocompetent people. Diagnosis of mycobacteria can be performed by microbiological culture from tissue samples (lymph nodes, lungs) and secretions (sputum, milk). In general, these pathogens demand special nutrient requirements for isolation/growth, and the use of selective and rich culture media. Indeed, within these genera, mycobacteria are classified as either fast- or slow-growth microorganisms. Regarding the latter ones, incubation times can vary from 45 to 90 days. Although microbiological culture is still considered the gold standard method for diagnosis, molecular approaches have been increasingly used. We describe here an overview of the diagnosis of Mycobacterium species in bovine milk.

A study of cattle-to-cattle transmission of Mycobacterium bovis infection

Twenty steers, positive to the single intradermal comparative tuberculin test (SICTT), were selected from herds with a recent history of Mycobacterium bovis infection. Ten steers, negative to SICTT, were selected from herds with no history of M. bovis infection and served as in-contact animals. The animals were divided into 10 groups, each consisting of two SICTT-positive (reactor) animals and one in-contact animal. Each group was housed in an individual loose-box for a period of 1 year. Five of the groups were fed a restricted diet for part of the experiment. All cattle were slaughtered at the end of the study period and examined at post mortem. Transmission of infection to an in-contact animal occurred in four of the 10 groups. One of the four in-contact animals, which became infected, had a retropharyngeal lymph node tubercle and M. bovis was isolated from lymph nodes without visible lesions from the other three. Two of the infected in-contact animals without visible lesions did not show any detectable cell-mediated immune response. There was no evidence that dietary, restriction had any effect on transmission of disease.

Post mortem diagnosis of Mycobacterium bovis infection in cattle

A tentative diagnosis of bovine tuberculosis can be made following the macroscopic detection at necropsy of typical lesions. Histo-pathological examination of the lesion may increase the confidence of the diagnosis but bacteriological isolation of Mycobacterium bovis from the lesion is the only way to make a definitive diagnosis. The sensitivity of gross post mortem examination is affected by the method employed and the anatomical sites examined. Careful examination of as few as 6 pairs of lymph nodes, the lungs and the mesenteric lymph nodes can result in 95% of cattle with macroscopic lesions being identified. Although during post mortem inspection of carcases at abattoir all the principle sites where lesions are to be found were examined, this procedure was found to be insensitive for the detection of lesions. To determine the significance of cattle that give a positive reaction in diagnostic tests but do not have visible lesions (NVL), a bacteriological examination is necessary. NVLs may be due to early infection, poor necropsy technique or infection with mycobacteria other than M. bovis. M. bovis was found to survive best in frozen tissue and the tissue preservative, sodium tetraborate, was found to have adverse effects on viability. It was found desirable to use two different culture media for the primary isolation of M. bovis; agar media for rapid growth and egg media for control of contamination. Additional control of contamination was achieved without adversely affecting the viability by treating the specimen before culture with 0.075% hexadecylpyridinium chloride. The addition of CO2 to the incubation atmosphere did not enhance the recovery of M. bovis. Conventional identification of isolates of M. bovis is by biochemical tests and cultural characteristics, but methods employing monoclonal antibodies and DNA probes may be used to obtain a rapid identification.

Advantages of a new agar medium in the primary isolation of Mycobacterium bovis

Suspect tuberculous lesions from 116 cattle were examined histologically and cultured for Mycobacterium bovis using 5 different media. The media used were: B83, an agar medium incorporating bovine blood and sodium pyruvate; Middlebrook's agar; 2 variations of Stonebrink's medium; Löwenstein-Jensen medium. The B83 medium and a modification of Stonebrink's medium which had a lowered concentration of malachite green were most successful, detecting 95.2% of tuberculous animals when used together. The B83 medium detected isolates approximately 1 week earlier and had more colonies than the Stonebrink's modification. A combination of 2 slopes of B83 and 2 slopes of modified Stonebrink's medium is recommended for routine culture of samples.

An evaluation of 1-hexadecylpyridinium chloride as a decontaminant in the primary isolation of Mycobacterium bovis from bovine lesions

For the primary isolation of Mycobacterium bovis from bovine lesions, 1-hexadecylpyridinium chloride (HPC) at a concentration of 0.75% was as effective as 2% NaOH in controlling the growth of contamination. The advantages of using HPC over NaOH are that it is a rapid one-step procedure not requiring neutralisation with acid, it is less toxic to M. bovis thus increasing isolation rates, and it promotes the earlier appearance of colonies.

Identification of Mycobacterium bovis isolates using a monoclonal antibody

A rapid immunoperoxidase slide assay for the identification of Mycobacterium bovis culture isolates is described. The monoclonal antibody used in this assay is specific for the M. tuberculosis complex of organisms. All M. bovis isolates tested, including 151 separate field isolates of M. bovis were positive as were 11 out of 12 M. tuberculosis strains and 4 out of 6 Bacillus Calmette Guerin (BCG) strains. One strain each of M. africanum and M. microti was negative. This assay provides a considerable improvement in both time and expense over the conventional methods of biochemical typing of M. bovis.

Comparison of media used for the primary isolation of Mycobacterium bovis by veterinary and medical diagnostic laboratories

Veterinary and medical laboratories engaged in the cultural diagnosis of bovine or human tuberculosis were requested to supply samples of the media that they routinely use for the primary isolation of M. bovis. Fourteen laboratories supplied 7 basic media types; these were Lowenstein-Jensen, Stonebrink's, modified Middlebrook 7H11 agar, tuberculosis bovine blood agar, egg yolk agar, Gerloff's egg and Herrold's egg yolk. Two strains of M. bovis were used to test the media, strain AN5, a glycerol-tolerant laboratory strain and M86/90 a glycerol-sensitive wildtype strain. AN5 grew well on all media with the exception of Herrold's and strain M86/90 did not grow on media containing glycerol and grew poorly on Herrold's medium. It is recommended that Lowenstein-Jensen with pyruvate (but without glycerol), Stonebrink's, modified Middlebrook 7H11 and tuberculosis bovine blood agar should be considered the media of choice for the primary isolation of M. bovis. Egg yolk agar also proved adequate for this purpose in the trial. This medium may be suitable for routine use but to date experience with its use is limited.

Experimental infection of badgers (Meles meles) with Mycobacterium bovis

The intradermal inoculation of four badgers with small numbers of Mycobacterium bovis resulted in localized lesions with ulceration which slowly healed by 5 months after inoculation. Lesions of generalized tuberculosis were seen in three badgers, one of which died at 17 months post-inoculation and in the remaining two killed 22 months post-inoculation. In the fourth badger lesions were confined to the draining lymph node of the inoculation site but M. bovis was isolated from the liver. Monthly clinical sampling of faeces, urine, tracheal aspirate and inoculation site exudates detected only the excretion of M. bovis from the inoculation site of one badger. There were marked seasonal variations in body weight but significant weight loss was observed during the second year in all four badgers, particularly prior to death. Four badgers inoculated intratracheally with a similar inoculum of M. bovis and another two control badgers showed no evidence of infection with M. bovis.

Tuberculosis in East Sussex. III. Comparison of post-mortem and clinical methods for the diagnosis of tuberculosis in badgers

Following epidemiological and ecological studies of a defined badger population in an area of East Sussex, removal of all badgers by cage trapping was attempted. Trapping was incomplete due to the activities of protesters. Forty-seven badgers were caught from the eight social groups. All badgers were examined clinically and samples of faeces, urine and tracheal aspirate were taken, together with swabs from any bite wounds, for bacteriological examinations. Forty-five animals were skin tested using whole killed cells of Mycobacterium bovis strain AN5, bovine PPD Weybridge and new human tuberculin. Skin test results were recorded after 24 and 72 h. All badgers were killed and subjected to a post-mortem and bacteriological examination. M. bovis was detected in 10 (21.3%) badgers at post-mortem and in 2 badgers from clinical samples. Four social groups were infected. Positive skin test results were recorded at 72 h with bovine PPD (2 micrograms and 20 micrograms/ml), strain AN5 (1 mg/ml) and human tuberculin (2 micrograms/ml), but not with human tuberculin at 20 micrograms/ml. Histological sections of the skin test reactions showed the cellular types typical of delayed-type hypersensitivity. The skin test reactions observed were neither sensitive nor specific enough to be of practical value.

Tuberculosis in East Sussex. II. Aspects of badger ecology and surveillance for tuberculosis in badger populations (1976-1984)

Following the disclosure of Mycobacterium bovis infection in badgers in East Sussex in 1976, badgers have been examined from and around farms on which cattle have become infected, but with no other attributable source of infection. These farms are confined to the downland of the south-west of the county and M. bovis has been confirmed in badger populations utilising their land. The available evidence indicates that M. bovis infection in badgers is also confined to this area. A detailed study in one area on the South Downs suggested that M. bovis is endemic in the badger population and therefore presents a continued risk for cattle occupying the area.

Tuberculosis in East Sussex. I. Outbreaks of tuberculosis in cattle herds (1964-1984)

The history and epidemiology of bovine tuberculosis in cattle herds in East Sussex are described. Since 1960, following the compulsory eradication scheme for tuberculosis, the incidence of herd infection has been low. The epidemiological features of herd infection have been sporadic incidents, with only small numbers of cattle becoming infected in the majority of incidents. There was no evidence of endemic Mycobacterium bovis infection in the cattle population in East Sussex in recent years, but a low risk of infection for cattle on the South Downs, from badgers, was apparent.

Bovine tuberculosis in domestic and wild mammals in an area of Dorset. II. The badger population, its ecology and tuberculosis status

Following a major outbreak of tuberculosis in cattle on a farm in Dorset, badgers were discovered to be infected with Mycobacterium bovis. Two hundred and forty sets were found in the 1200 hectares of the study area. The sets were found predominantly in areas of Portland Sand. A high prevalence of tuberculosis was found in the badger population which was removed and repopulation prevented for 3 years. The removal of the infected badgers led to the resolution of the problem in cattle. Re-colonization of the area has progressed slowly and the cattle have remained free from infection for a period of 5 years.

Tuberculosis in wildlife in the Ruwenzori National Park Uganda (part I)

The technique adopted for a survey of tuberculous infection in wild Cape buffalo (Syncerus caffer, Sparrman) living in the Ruwenzori National Park, Uganda is described and the results of gross post-mortem examinations of a random sample of 52 buffalo and a selected sample of 64 thin buffalo are tabulated. Tuberculosis was found to occur in buffalo in the area of the Park north of the Maramagambo Forest; the disease is considered to be absent from the area south of the forest. Tuberculosis was found to affect 10% of the random sample of buffalo and 38% of the selected thin sample. Laboratory procedures carried out for the isolation and typing of the mycobacteria are described. Mycobacterium bovis was shown to be responsible for 12 of 14 infections and atypical mycobacteria for the remaining two. The distribution and character of the lesions are described and an account of the epidemiology, morbidity and mortality of the disease is given. It is estimated that tuberculosis is responsible for an annual mortality of about 1% of the 18,000 buffalo inhabiting the Park.

A selective oleic acid albumin agar medium for the cultivation of Mycobacterium bovis

The modification of Middlebrook's 7H11 oleic acid albumin agar medium by the addition of fresh bovine serum and lysed sheep red cells to encourage growth of Mycobacterium bovis is described. The improved medium was made selective by the addition of antibiotics and a comparative trial of this medium and the guineapig test in the isolation of M. bovis from badger tissues is reported. A close agreement between the two tests was found; the guinea-pig test detected 95% of all isolations and culture detected 91%.