Tumble operation. A unit length random direction, say vector Δ(i) ∈ [−1,1], is generated for each bacterium and this defines the direction of movement after a move. The update of bacteria is mainly according to Eq 13. Q(i,t+1,k)=Q(i,t,k)+H(t,k)Δ(i)Δ(i)TΔ(i),(13) where Q(i,t,k) is the code of the kth position of bacterial i at iteration t, H(t, k) is the step size of direction for position k, which is generated randomly.