Several reports on changes in secondary metabolism during rotary wall vehicle suspension culture exist 
, however, they do not differentiate the effects of rotary force and microgravity. Valles and Guevorkian (2002) have described the use of magnetic field gradient levitation apparatus as a low gravity simulator for biological systems 
. Diamagnetic levitation certainly has the potential to be a powerful tool to study the effects of weightlessness on biological samples, to complement existing ground-based techniques, however artifacts may exist.
In this study, we have investigated, for the first time, the individual effects of magnetic field and gravitational force on the morphology and secondary metabolism of S. avermitilis
. From our experimental data, we discovered that the physiological response of strain PE1 to magnetic field exposure resulted in suppression of sporulation and a reduction in mycelium at 12T. There have been few reports on the effect of magnetic field on microbe growth. Zhang, et al. (2002) observed that the growth of E. coli
was inhibited due to the presence of magnetic field of up to 0.6 T, the maximum used in that experiment 
. Iwasakaa et al. (2004) observed that the rate of yeast proliferation decreased after 16 h of incubation under a high magnetic field (14T) compared to the control group 
. Ji et al. (2010) also found that high-level magnetic field (>200 mT) inhibited the growth of mixed bacteria of activated sludge 
. While these results confirm our own findings, mechanisms that explain how magnetic fields may initiate changes in biological systems have not yet been elucidated.
Other reports have focused on the mutagenic effects of static magnetic fields. Four strains of Salmonella typhimurium
(TA98, TA100, TA1535 and TA1537) and Escherichia coli
were exposed to a high magnetic field (5T) 
. The uvrA,
which lacked the UvrA protein that functions in the initial step of nucleotide excision repair but had normal activity to protect the cells against oxidative stress showed no mutagenic potential 
. Zhang, et al (2003) examined the effect of strong static magnetic field (up to 10T) on various E. coli
mutants defective in repair of oxidative DNA damage, redox regulation, defence systems against oxidative stress. Only the mutation in the mutants defective in defence mechanisms against oxidative stress was significantly enhanced in an exposure-dependent manner 
. In this work, we detected the mutation effect of a high magnetic field on S. avermitilis
strain PE1. Streptomyces
lives in the soil and are often challenged with diverse environmental stresses 
which always trigger cell morphological differentiation associated with secondary metabolism 
. The strain PE1, used in this study, was obtained by serially random mutation in order to obtain a high titer of avermectin production. The mutation effect of the magnetic field on strain PE1 suggests that, the defense mechanism for oxidative stress or other stress factors in strain PE1 might have been disabled by these purposeful mutations prior to this experiment.
Results presented here suggest that the effects of gravity can be differentiated from the effects of magnetic field alone. This study showed that with diamagnetic levitation, magnetic field, rather than gravity, is the dominant factor inducing physiological response in strain PE1. The authors thus suggest caution to be used when analyzing results where diamagnetic levitation is used to simulate a microgravity environment.