Electron micrographs of magnetotactic bacteria reveal that chains of magnetosomes are often bent. This is surprising inasmuch as straight chains are actually the most favourable arrangement for magnetonavigation achieving the maximum value of the bacterial net magnetic moment. In order to answer the question of what causes the chains to bend, we calculated the stability limit of straight magnetosome chains by taking into account elastic and magnetic forces. For several scenarios, the threshold values of external forces leading to elastic instability were computed. From our calculations and observations on freeze-dried cells, we conclude that, under normal conditions, magnetosome chains are straight or only slightly bent, whereas shrinkage during preparation may cause severe artifacts such as kinks or zig-zag structures in the chains.