Up to 44 percent of strains of Streptococcus pyogenes and 74 percent of Streptococcus faecalis have been found to be resistant to tetracycline drugs. Therefore, tetracycline should not be used for streptococcal disease unless the organism has been demonstrated to be susceptible. Treponema pallidum Balantidium coli Treponema pertenue Plasmodium falciparum Doxycycline has been found to be active against the asexual erythrocytic forms of plasmodium falciparum but not against the gametocytes of P.falciparum . The precise mechanism of action of the drug is not known. Quantitative methods that require measurement of zone diameters give the precise estimate of the susceptibility of bacteria to antimicrobial agents.
Standardized procedures require the use of laboratory control organisms.
Resistant As with standard diffusion techniques, dilution methods require the use of laboratory control organisms. A tetracycline disk may be used to determine microbial susceptibility. This goitrogenic effect was accompanied by high radioactive iodine uptake. Administration of minocycline also produced a large goiter with high radioiodine uptake in rats fed a relatively high iodine diet. Adrenal gland hyperplasia has been observed in goats and rats treated with oxytetracycline.