1. Summary The proportion of carrier‐isolated Neisseria meningitidis strains sensitive to human serum (37.2%) was found to be significantly higher than that of case‐isolated ones (4.1%), although the difference is too low to consider serum‐resistance responsible for invasion in this microorganism. Serum‐susceptibility was not related to the existence of specific outer membrane proteins, as is the case in N. gonorrhoeae. Iron restriction induced iron‐regulated outer membrane proteins in each strain (but not the same proteins in all strains) but without any detectable effect on serum‐susceptibility. Iron excess was also unable to induce changes in the susceptibility of N. meningitidis to human serum.

The influence of surface charge on hydrophobicity and adherence of four strains of Neisseria meningitidis was studied. After neutralization of negative (treatment with EDC-methylamine), positive (treatment with formaldehyde) or both charges, adherence to buccal cells and hydrophobicity were determined and analysed. It was found that surface charges are relevant for adherent processes. Surface hydrophobicity was also found to be influenced by charge but uncorrelated with adherence.

The study of surface hydrophobicity and adherence to human buccal epithelial cells of four Neisseria meningitidis strains and the correlation analysis of the data obtained for both parameters has demonstrated that hydrophobicity of meningococci is not a causative factor in adherence in normal conditions. Modification of the bacterial surfaces (by freezing, heating, ultraviolet irradiation, fixation with glutaraldehyde or sodium metaperiodate cleavage of external sugars) induced significant changes in one or both parameters, but these changes were not correlative in both properties. In contrast to other bacteria, the hydrophobicity levels shown by our meningococci are not relevant to their adherent processes.

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Surface hydrophobicity and adherence to buccal epithelial cells were studied in 33 carrier and 34 invasive Neisseria meningitidis strains. It was found that hydrophobicity is statistically similar in both groups (P = 0.0507) although it could be considered that carrier strains are slightly more hydrophobic than invasive ones. Adherence was similar in both groups although more homogeneous in the carrier strains. No correlation could be demonstrated between these two properties nor can they be considered relevant as markers of the carrier or invasive status of this bacterium, indicating that at least in N. meningitidis they are not good properties to discriminate virulent strains.