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Addition of isovitalex in chocolate agar for the isolation of Haemophilus influenzae.

Background & objectives: The reason for lack of data on burden ofHaemophilus influenzae type b (Hib) in developing countries was mainly failure of detection of this fastidious organism in laboratories. Use of isovitalex (IVX) was suggested as an essential supplement for growing this organism. This study was carried out to investigate the impact of 1VX supplementation to chocolate agar for detection of Hib.

Methods: Chocolate agar with and without supplementation of IVX was prepared. Clinical samples as well as reference strains of Hib were simultaneously cultured on both the media.

Results: H. influenzae isolates (N=194) were simultaneously grown on chocolate agar (CA) with and without isovitalex (IVX). Average colony size of H. influenzae on CA with IVX (CA-IVX) was larger only by 0.10 cm (range 0.05 to 0.16 cm) compared to CA alone. Addition of IVX to CA increased the cost of media by 2.1-fold.

Interpretation & conclusion: Isovitalex is not essential for the isolation and growth of H. influenzae almost halving the cost.

Key words Growth--Haemophilus influenzae--isovitalex


Benefits of Haemophilus influenzae type b (Hib) vaccine are primarily limited to children of the developed part of the world mainly for lack of convincing local disease burden data from the developing countries. A principal reason for the underestimation of Hib is the failure of most laboratories in developing countries, especially in Asia, to detect this fastidious micro organism (1-3).

Hib has, appeared as a predominant cause of meningitis and pneumonia whenever explored applying standard microbiological procedures and rigorous quality controls (4-6). However, use of special base media and/or supplement(s) is being recommended for investigating Hib burden, instead of emphasizing on good laboratory practices mentioned in the text books (7,8). In 1994, Gellert et al (1) reported isovitalex (IVX) as an essential nutrient for the growth of H. influenzae and that lack of this supplement in the culture medium led to failure of the Latvian National Bacteriology Laboratory to isolate the organism. They also reported that plain chocolate agar (CA) could not support growth of H. influenzae on subculture, unless supplemented with IVX. This suggestion has been well taken by the public health practitioners and policy makers, and the addition of IVX as an essential supplement for the isolation of H. influenzae has been included as a standard procedure in the laboratory manuals (9). It is now widely believed that isolation of H. influenzae is difficult and requires substantial resources (2), which has served to discourage laboratories of many developing countries from exploring the burden of Hib diseases.

We therefore used plain CA and IVX-supplemented (1%) CA (CA-IVX) simultaneously to isolate H. influenzae and other fastidious organisms, Neisseria meningitidis and Streptococcus pneumoniae, and compared for their colony size. Comparative cost of CA with and without IVX were also calculated and compared.

Material & Methods

The study was conducted at the Department of Microbiology of Dhaka Shishu Hospital, Dhaka, Bangladesh; during the period of 1995-1998. CA was prepared by heating blood agar base (Oxoid Ltd. Hampshire, England) with 5 per cent sheep blood at 75-80[degrees]C for 15 min in a water bath. IVX (Becton Dickinson, Sparks, USA) was added to CA at 45[degrees]C, and equal amounts (10 ml) of CA or CA-IVX were dispensed into respective separate chambers of the same petriplate. Each batch of CA media with and without supplement was tested for adequate growth of reference strain H. influenzae ATCC 49247. Cerebrospinal fluid (CF) specimens were collected from the children with suspected meningitis and sent to the department of microbiology as part of their clinical care. The specimens were simultaneously cultured on both media on the same plate and incubated at 37[degrees]C in a CO2 jar for 18-20 h, following the standard procedure (8). In addition to culturing the clinical specimens, Hib isolates collected from meningitis and pneumonia case of Japan (n=5) and the USA (n=10) were also cultured on both the media. Temperature and time of incubation were identical for both type of plates, and laboratory personnel remained blinded to the media type while culturing and observing bacterial growth. Impact of IVX on colony size of H. influenzae was tested by culturing them on CA, with and without IVX. After 18-20 h of incubation in CO2 jar, isolated colonies of the same isolate, at the distant part of streaking, on each of the media were digitalized and the average of twoway diameters of a colony was taken with the scale generated by Photoshop Adobe Program Version 6.0 (Adobe systems Incorporated, Sanjose, CA, USA). Diameters of 5 separate colonies of the same isolate on each media were measured in the same way and average value calculated.

Results & Discussion

During the period of 1995 to 1998, 540 CSF specimens were simultaneously cultured on both the media and 194 H. influenzae isolates were obtained. Of these, 189 were type b, 2 were type a and 3 were nontypable. All strains were isolated on both CA and CA-IVX after the same period of incubation, irrespective of their serotype and biotype. The ATCC strain 49247 always showed confluent growth on both the media. Although Oxoid blood agar base was mainly used in this study, we transiently switched to blood agar base of Remel (Lenexa, USA), and Mast Diagnostics (Merseyside, UK), and found that the CA made from any of these basal media, either with or without IVX supplementation, supported confluent growth of H. influenzae. All isolates of H. influenzae collected from Japan. and the USA also grew well on CA and CA-IVX.

Average colony size of H. influenzae on CAIVX was larger by 0.10 cm (range 0.05 to 0.16 cm) compared to CA. Calculation of cost showed that addition of isovitalex to CA increased its price by 2.1-fold.

One of the major reasons for the low usage of Hib vaccines particularly in developing countries, is the perception that Hib is not an important cause of disease (2,6). This perception is a result of low isolation rates of Hib in developing countries, in part due to poor laboratory facilities and limited resources. It is, therefore, important to use methods that are inexpensive as well as sensitive for the detection of Hib.

We have successfully grown all 194 H. influenzae isolates on plain CA, irrespective of their sero- or biotype, without the addition of IVX. This indicates that CA is sufficient to support the growth of this organism.

Failure of other laboratories in Bangladesh (3), India (10), and Guatemala (Dr Edwin Esturias, JHU, personal communication) to isolate this fastidious organism was possibly due to faulty preservation of specimens before culturing, use of poor quality base media or source of blood, faulty preparation of media or a combination of these factors. Our success in isolating all H. inlfuenzae isolates further emphasizes that the most important factors in preparing media for isolation of this organism are to (i) use a good quality and appropriate basal medium; (ii) addition of blood from an animal source (not human blood); (iii) use of correct temperature and time to chocolatise the media; (iv) check the quality of each batch for its optimum ability to grow a set of test strains; and (v) use the prepared plates in a timely fashion. Minimal difference in colony diameter (0.10 cm) and growth of all isolates of 11. influenzae indicates that IVX is not essential for primary isolation of this organism.

In conclusion, we recommend that isovitalex is not essential for the growth of H. influenza, and the resource-poor institutions, with good laboratory practice, can continue their effort to isolate Hib without wasting their resources for IVX.


The study was partially funded by Ministry of Health and Labor of Japan. Authors acknowledge Ms. Tania Nasreen for technical support and Prof. Gary Doem, University of Iowa, USA, for providing the strains.

Received September 11, 2007


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(9.) Levine OS, Schuchat A, Schwartz B, Wenger JD, Elliott J. Genetic protocol for population based surveillance to assess the local burden of meningitis due to Haemophilus influenzae type b among children less than 5 years old. Geneva: World Health Organization (WHO/VRD/GEN/95.05); 1996.

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Reprint requests: Dr Samir K. Saha, Professor & Head, Department of Microbiology, Dhaka Shishu (Children) Hospital Bangladesh Institute of Child Health, Dhaka 1207, Bangladesh e-mail:

S.K. Saha *,**, A.H. Baqui **, (++), G.L. Darmstadt **, M. Islam *, S.E. Arifeen (++), M. Santosham **, T. Nagatake ([dagger]) & R.E. Black **

* Department of Microbiology, Dhaka Shishu (Children) Hospital, Dhaka, Bangladesh Institute of Child Health, Dhaka, Bangladesh; ** Department of International Health, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA; (++) Child Health Programme, ICDDR, B, Dhaka, Bangladesh & ([dagger]) Department of Internal Medicine, Institute of Tropical Medicine, Nagasaki University, Nagasaki, Japan
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Author:Saha, S.K.; Baqui, A.H.; Darmstadt, G.L.; Islam, M.; Arifeen, S.E.; Santosham, M.; Nagatake, T.; Bla
Publication:Indian Journal of Medical Research
Article Type:Report
Geographic Code:9INDI
Date:Jan 1, 2009
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