Magnetotactic bacteria form chains of magnetite nanoparticles that serve the organism as navigation tools. The magnetic anisotropy of the superstructure makes the chain an ideal model to study the magnetic properties of such an organization. Magnetic force microscopy (MFM) is currently the technique of choice for the visualization of magnetic nanostructures, however it does not enable the quantitative measurement of magnetic properties, since the interactions between the MFM probe and the magnetic sample are complex and not yet fully understood. Here we present an MFM study of such a chain of biological magnetite nanoparticles. We combined experimental and theoretical (Monte Carlo simulation) analyses of the sample, and investigated the size and orientation of the magnetic moments of the single magnetic particles in the chain. MonteCarlo simulations were used to calculate the influence of the magnetic tip on the configuration of the sample. The advantage of this procedure is that analysis does not require any a priori knowledge of the properties of the sample. The magnetic properties of the tip and of the magnetosomes are indeed varied in the calculations until the phase profiles of the simulated MFM images achieve a best match with the experimental ones. We hope our results will open the doors towards a better quantification of MFM images, and possibly a better understanding of the biological process in situ.