Various methods have been developed to detect horizontal gene transfer in 
bacteria, based on anomalous nucleotide composition, assuming that compositional 
features undergo amelioration in the host genome. Evolutionary theory predicts 
the inevitability of false positives when essential sequences are strongly 
conserved. Foreign genes could become more detectable on the basis of their 
higher order compositions if such features ameliorate more rapidly and uniformly 
than lower order features. This possibility is tested by comparing the 
heterogeneities of bacterial genomes with respect to strand-independent first- 
and second-order features, (i) G + C content and (ii) dinucleotide relative 
abundance, in 1 kb segments. Although statistical analysis confirms that (ii) is 
less inhomogeneous than (i) in all 12 species examined, extreme anomalies with 
respect to (ii) in the Escherichia coli K12 genome are typically co-located with 
essential genes.