Tissue samples collected at necropsy from cattle suspected of being infected with bovine tuberculosis (TB) are submitted to an appropriate animal diagnostic laboratory for the attempted culture of Mycobacterium bovis. Because of the importance of bovine TB, these labs follow strict procedures to ensure the accuracy of their culture methods. Culturing for Mycobacterium bovis is performed under specific conditions that favor the growth of the bacteria. Unfortunately, Mycobacterium bovis are very slow growing organisms and can take 8 to 16 weeks to grow. Therefore, a culture is not called negative for bovine TB until after it has been incubated for 3 to 4 months. If an organism is isolated from submitted tissue samples, it is subject to further testing to determine if the isolate is Mycobacterium bovis or some other closely related Mycobacterium species. This is done using biochemical and genetic testing (see PCR and DNA Fingerprinting sections).